Make GPU optimal_learning components selectable from the REST interface

[suntzu86]

Through #368 and #394, we can now call GPU functions and conduct EI optimization on the GPU via the python interface (assuming you have a GPU and built the MOE-GPU components; if not, we raise exceptions).

We should make this functionality selectable from the REST interface which requires allowing users to select GPU mode (true/false) and indicate which GPU(s) to use (integers). So let's define a schema and add it to the params for the EI computation and EPI optimization views.