Jialei gh219 improve consistency check between gpu vs cpu computation for general qp ei

********* PEOPLE *************
Primary reviewer: @suntzu86

Reviewers: @sc932 @ypwais @yaoster

********* DESCRIPTION **************
Branch Name: jialei_gh219_improve_consistency_check_between_gpu_vs_cpu_computation_for_general_qp_EI
Ticket(s)/Issue(s): Closes #219 

********* TESTING DONE *************
make test-no-tox: total test time 27.40s
cpplint.py to check style
docstring