Hide boost rng behind facade for osx compatibility

Split boost random number generation from the common Caffe singleton and
add a helper function for rng. This resolves a build conflict in OSX
between boost rng and nvcc compilation of cuda code.

Refer to #165 for a full discussion.

Thanks to @satol for suggesting a random number generation facade rather
than a total split of cpp and cu code, which is far more involved.
Set copyright to BVLC and contributors.

The exact details of the contributions are recorded by versioning.