add Adopt{Data,Diff} methods to blobs to enable "virtual copying"
change Adopt -> Share as suggested by kloudkl
sigmoid layer backward pass optimization: don't recompute forward pass
Merge pull request #319 from jeffdonahue/sigmoid-optimization

sigmoid layer backward pass optimization: don't recompute forward pass
Merge pull request #318 from jeffdonahue/blob-copy-by-reference

add Share{Data,Diff} methods to blobs to enable "virtual" copies

- reduce operations by not copying values
- spare memory by shared_ptrs
- simplify SplitLayer logic
Add possibility to use OpenBlas
Merge pull request #325 from AlOa/OpenBlas

Add possibility to use OpenBlas
rename python include config var to match lib
Give choice of ATLAS, MKL, and OpenBLAS (with option to override paths)

- configure build for ATLAS, MKL, or OpenBLAS on Linux and OSX
- allow overriding of the include or lib dirs
- replace magic numbers with BLAS names (atlas, mkl, open)

Follow-up from #305 and #325.
rename lenet dir to mnist
add mnist autoencoder example necessities (sigmoid cross entropy loss
layer, sparse gaussian filler)
enable DataLayer to output unlabeled data
mnist autoencoder test proto bugfix: add sigmoid layer before loss
make solver able to compute and display test loss
change lenet dir to 'mnist' in docs
mnist_autoencoder_solver cleanup
add sigmoid cross ent layer unit tests
clear sigmoid top vec at initialization
change to correct next layer id for merge
Merge pull request #330 from jeffdonahue/mnist-autoencoder-example

MNIST autoencoder example