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
there needs to be a newline btwn the last text and param, e.g.,
```
blah blah blah

\param
  :foo: stuff
\output
  :bar: more stuff
```
o/w sphinx gets confused