Added a class for GPU matrices

Added an SGMatrix-like class for GPU matrices. It supports conversion to and from SGMatrix and Eigen3 matrices, wrapping a matrix around pre-allocated memery, and serialization.

I've also wanted to add methods to SGMatrix that convert to and from Eigen3 matrices, however it fails to compile on travis for some reason.

@lisitsyn, @lambday, @karlnapf Please take a look.
This part and its corresponding unit test compile and run successfully on my machine, however it fails to compile on travis for some reason ([log](https://travis-ci.org/khalednasr/shogun/jobs/28917222#L1971)).