Print blob debug info during training if SolverParameter "debug_info" field is set

(Heavily inspired by the venerable [cuda-convnet](http://code.google.com/p/cuda-convnet/wiki/TrainingNet).)

This PR lets you add "debug_info: true" to your solver.prototxts to compute and print debug information during the forward pass, backward pass, and updates (only prints on "display" iterations when the training loss is normally displayed).   Specifically, the information that's printed is L1 norms (sum of absolute values) followed by the mean absolute value in parentheses.  This can be helpful for debugging issues with training.  For example, if you see a nan/inf loss, this would let you figure out which layer is producing the first nan/inf and when.  Here's what the output looks like when you add `debug_info: true` to the end of `lenet_consolidated_solver.prototxt`:

```
I0726 14:56:33.147119 27342 solver.cpp:143] Iteration 0, Testing net (#0)
I0726 14:56:35.349648 27342 solver.cpp:181] Test score #0: 0.067
I0726 14:56:35.349707 27342 solver.cpp:181] Test score #1: 2.30256
I0726 14:56:35.349717 27342 solver.cpp:143] Iteration 0, Testing net (#1)
I0726 14:56:41.799576 27342 solver.cpp:181] Test score #0: 0.0670334
I0726 14:56:41.799634 27342 solver.cpp:181] Test score #1: 2.30258
I0726 14:56:44.788625 27342 net.cpp:424]     [debug_info] [Forward] Layer mnist, top blob data data: 7065.09 (0.140806)
I0726 14:56:44.790845 27342 net.cpp:424]     [debug_info] [Forward] Layer mnist, top blob label data: 290 (4.53125)
I0726 14:56:44.795804 27342 net.cpp:424]     [debug_info] [Forward] Layer conv1, top blob conv1 data: 227884 (0.309087)
I0726 14:56:44.798096 27342 net.cpp:424]     [debug_info] [Forward] Layer pool1, top blob pool1 data: 64825.6 (0.351701)
I0726 14:56:44.807307 27342 net.cpp:424]     [debug_info] [Forward] Layer conv2, top blob conv2 data: 194818 (0.951262)
I0726 14:56:44.809547 27342 net.cpp:424]     [debug_info] [Forward] Layer pool2, top blob pool2 data: 53716 (1.04914)
I0726 14:56:44.811975 27342 net.cpp:424]     [debug_info] [Forward] Layer ip1, top blob ip1 data: 21170.2 (0.661568)
I0726 14:56:44.814205 27342 net.cpp:424]     [debug_info] [Forward] Layer relu1, top blob ip1 data: 16661.7 (0.520679)
I0726 14:56:44.816563 27342 net.cpp:424]     [debug_info] [Forward] Layer ip2, top blob ip2 data: 2039.79 (3.18717)
I0726 14:56:44.819965 27342 net.cpp:439]     [debug_info] [Backward] Layer loss, bottom blob ip2 diff: 0.378046 (0.000590697)
I0726 14:56:44.822183 27342 net.cpp:439]     [debug_info] [Backward] Layer ip2, bottom blob ip1 diff: 2.73874 (8.55855e-05)
I0726 14:56:44.824498 27342 net.cpp:449]     [debug_info] [Backward] Layer ip2, param blob 0 diff: 50.081 (0.0100162)
I0726 14:56:44.826732 27342 net.cpp:449]     [debug_info] [Backward] Layer ip2, param blob 1 diff: 0.168365 (0.0168365)
I0726 14:56:44.829035 27342 net.cpp:439]     [debug_info] [Backward] Layer relu1, bottom blob ip1 diff: 1.87708 (5.86588e-05)
I0726 14:56:44.831616 27342 net.cpp:439]     [debug_info] [Backward] Layer ip1, bottom blob pool2 diff: 2.14405 (4.1876e-05)
I0726 14:56:44.833940 27342 net.cpp:449]     [debug_info] [Backward] Layer ip1, param blob 0 diff: 527.024 (0.00131756)
I0726 14:56:44.836174 27342 net.cpp:449]     [debug_info] [Backward] Layer ip1, param blob 1 diff: 0.585842 (0.00117168)
I0726 14:56:44.838559 27342 net.cpp:439]     [debug_info] [Backward] Layer pool2, bottom blob conv2 diff: 2.14405 (1.0469e-05)
I0726 14:56:44.850530 27342 net.cpp:439]     [debug_info] [Backward] Layer conv2, bottom blob pool1 diff: 6.14938 (3.33625e-05)
I0726 14:56:44.852756 27342 net.cpp:449]     [debug_info] [Backward] Layer conv2, param blob 0 diff: 55.5542 (0.00222217)
I0726 14:56:44.855058 27342 net.cpp:449]     [debug_info] [Backward] Layer conv2, param blob 1 diff: 0.189715 (0.00379431)
I0726 14:56:44.857542 27342 net.cpp:439]     [debug_info] [Backward] Layer pool1, bottom blob conv1 diff: 6.14938 (8.34063e-06)
I0726 14:56:44.877032 27342 net.cpp:449]     [debug_info] [Backward] Layer conv1, param blob 0 diff: 3.22905 (0.0064581)
I0726 14:56:44.879184 27342 net.cpp:449]     [debug_info] [Backward] Layer conv1, param blob 1 diff: 0.21928 (0.010964)
I0726 14:56:44.879220 27342 solver.cpp:276] Iteration 100, lr = 0.00992565
I0726 14:56:44.882715 27342 net.cpp:466]     [debug_info] [Update] Layer conv1, param 0 data: 94.5529 (0.189106); diff: 0.101136 (0.000202272)
I0726 14:56:44.887137 27342 net.cpp:466]     [debug_info] [Update] Layer conv1, param 1 data: 1.17435 (0.0587177); diff: 0.00966474 (0.000483237)
I0726 14:56:44.890619 27342 net.cpp:466]     [debug_info] [Update] Layer conv2, param 0 data: 996.6 (0.039864); diff: 1.48126 (5.92504e-05)
I0726 14:56:44.894016 27342 net.cpp:466]     [debug_info] [Update] Layer conv2, param 1 data: 0.499523 (0.00999046); diff: 0.00582091 (0.000116418)
I0726 14:56:44.897387 27342 net.cpp:466]     [debug_info] [Update] Layer ip1, param 0 data: 1194.18 (0.00298545); diff: 14.1294 (3.53234e-05)
I0726 14:56:44.900827 27342 net.cpp:466]     [debug_info] [Update] Layer ip1, param 1 data: 1.96221 (0.00392441); diff: 0.0264453 (5.28906e-05)
I0726 14:56:44.904175 27342 net.cpp:466]     [debug_info] [Update] Layer ip2, param 0 data: 134.482 (0.0268964); diff: 1.19755 (0.00023951)
I0726 14:56:44.907567 27342 net.cpp:466]     [debug_info] [Update] Layer ip2, param 1 data: 0.668925 (0.0668925); diff: 0.00728625 (0.000728625)
I0726 14:56:44.907627 27342 solver.cpp:116] Iteration 100, loss = 0.308106
I0726 14:56:47.179862 27342 net.cpp:424]     [debug_info] [Forward] Layer mnist, top blob data data: 6074.86 (0.121071)
I0726 14:56:47.179925 27342 net.cpp:424]     [debug_info] [Forward] Layer mnist, top blob label data: 286 (4.46875)
I0726 14:56:47.181391 27342 net.cpp:424]     [debug_info] [Forward] Layer conv1, top blob conv1 data: 220081 (0.298503)
I0726 14:56:47.181501 27342 net.cpp:424]     [debug_info] [Forward] Layer pool1, top blob pool1 data: 61939.6 (0.336044)
I0726 14:56:47.184751 27342 net.cpp:424]     [debug_info] [Forward] Layer conv2, top blob conv2 data: 181365 (0.885573)
I0726 14:56:47.184839 27342 net.cpp:424]     [debug_info] [Forward] Layer pool2, top blob pool2 data: 42201.8 (0.824253)
I0726 14:56:47.185045 27342 net.cpp:424]     [debug_info] [Forward] Layer ip1, top blob ip1 data: 21178.1 (0.661815)
I0726 14:56:47.185094 27342 net.cpp:424]     [debug_info] [Forward] Layer relu1, top blob ip1 data: 13912.3 (0.434759)
I0726 14:56:47.185168 27342 net.cpp:424]     [debug_info] [Forward] Layer ip2, top blob ip2 data: 2183.11 (3.4111)
I0726 14:56:47.185251 27342 net.cpp:439]     [debug_info] [Backward] Layer loss, bottom blob ip2 diff: 0.362956 (0.000567119)
I0726 14:56:47.185317 27342 net.cpp:439]     [debug_info] [Backward] Layer ip2, bottom blob ip1 diff: 3.45429 (0.000107947)
I0726 14:56:47.185358 27342 net.cpp:449]     [debug_info] [Backward] Layer ip2, param blob 0 diff: 37.824 (0.0075648)
I0726 14:56:47.185397 27342 net.cpp:449]     [debug_info] [Backward] Layer ip2, param blob 1 diff: 0.162711 (0.0162711)
I0726 14:56:47.185441 27342 net.cpp:439]     [debug_info] [Backward] Layer relu1, bottom blob ip1 diff: 2.06665 (6.45827e-05)
I0726 14:56:47.185824 27342 net.cpp:439]     [debug_info] [Backward] Layer ip1, bottom blob pool2 diff: 2.93497 (5.73236e-05)
I0726 14:56:47.185894 27342 net.cpp:449]     [debug_info] [Backward] Layer ip1, param blob 0 diff: 523.536 (0.00130884)
I0726 14:56:47.185935 27342 net.cpp:449]     [debug_info] [Backward] Layer ip1, param blob 1 diff: 0.776996 (0.00155399)
I0726 14:56:47.186048 27342 net.cpp:439]     [debug_info] [Backward] Layer pool2, bottom blob conv2 diff: 2.93497 (1.43309e-05)
I0726 14:56:47.191087 27342 net.cpp:439]     [debug_info] [Backward] Layer conv2, bottom blob pool1 diff: 8.45792 (4.58872e-05)
I0726 14:56:47.191174 27342 net.cpp:449]     [debug_info] [Backward] Layer conv2, param blob 0 diff: 58.7563 (0.00235025)
I0726 14:56:47.191213 27342 net.cpp:449]     [debug_info] [Backward] Layer conv2, param blob 1 diff: 0.200319 (0.00400639)
...
```