Adadelta

Initial implementation of the Adadelta solver as proposed in "ADADELTA: An Adaptive Learning Rate Method" (Zeiler, 2012). Motivation: http://cs.stanford.edu/people/karpathy/convnetjs/demo/trainers.html

Performance on the MNIST autoencoder demo is more or less on par with standard SGD+momentum but not as good as the Nesterov solver. The lack of a learning rate does seem to be a problem towards later iterations in that loss/accuracy don't entirely converge, but this could be due to an implementation issue.

```
Iteration 64500, Testing net (#0)
Test loss: 59.4627
    Test net output #0: cross_entropy_loss = 59.4627 (* 1 = 59.4627 loss)
    Test net output #1: l2_error = 1.82881
Iteration 64500, Testing net (#1)
Test loss: 59.7422
    Test net output #0: cross_entropy_loss = 59.7422 (* 1 = 59.7422 loss)
    Test net output #1: l2_error = 1.92399
Iteration 65000, loss = 62.1569
Iteration 65000, Testing net (#0)
Test loss: 60.7756
    Test net output #0: cross_entropy_loss = 60.7756 (* 1 = 60.7756 loss)
    Test net output #1: l2_error = 2.05861
Iteration 65000, Testing net (#1)
Test loss: 61.0705
    Test net output #0: cross_entropy_loss = 61.0705 (* 1 = 61.0705 loss)
     Test net output #1: l2_error = 2.15354
```

A couple of things to note: 
- Adadelta requires the tracking of both gradient and update history. I chose to store both sequentially in "history_" for a couple of reasons, e.g. to reuse SnapshotSolverState() and RestoreSolverState.
- All the tests pass, but a couple (those with multiple iterations) are ridiculously slow, even though the MNIST demo for example is not noticeably slower with Adadelta compared to the other solvers. I still need to look into this.