Revert "Merge pull request #114 from jeffdonahue/dags-by-split"

Revert dags-by-split merge because split layers were not quite ready.
Sorry for the bug!

We are adopting a `master`/`dev` split development
model, where new commits and pull requests will be merged to `dev` for
testing before integration to `master`. This will keep master clean and
stable.

This reverts commit d339d242b43ec5bec9dceb657ee0d665d524b1eb, reversing
changes made to 55198262d79944fa7382868f7ecd108ed4f238a2.
Merge remote-tracking branch 'upstream/master'
infrastructure for low-mem conv forward GPU