Add split dim layer

Differentiate top test blob vector size

Rename to SplitLayer
Add slicing points
SliceLayer: post-rebase fixes, cleanup, etc. (some from changes
suggested by @sguada).  Test for both num & channels in forward &
backward; use GaussianFiller so that tests are non-trivial.
Merge pull request #755 from jeffdonahue/bhack-split_dim

Finishing touches on @bhack's SliceLayer (#680)