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
two more:

1. ```&mu_local[num_union]``` is the same as ```mu_local + num_union``` and the latter is more clear imo

2. idx is fixed right? Why not set ```normals = mu_local + num_union + idx * num_union```? also again be very specific about the ordering of these matrices in shared memory