Jialei gh297 speed up gpu computation on ei grad ei

********* PEOPLE *************
Primary reviewer: @suntzu86 

Reviewers: @sc932 

********* DESCRIPTION **************
Branch Name: jialei_gh297_speed_up_gpu_computation_on_EI_gradEI
Ticket(s)/Issue(s): Closes #297 

********* TESTING DONE *************
make test
cpplint.py
also have you ever worked with "constant" memory? i'm not sure if that's appropriate for mu and chol_var--yes they are constant, but gpu constant memory has some additional requirements on how a warp of threads accesses the data to get the best performance.

still if it's usable that could reduce some memory pressure