Hook up BFGS to rest

********* PEOPLE *************
Primary reviewer: @sc932 

Reviewers: @suntzu86 

********* DESCRIPTION **************
Branch Name: denizokt_285_qei_rest
Ticket(s)/Issue(s): Closes #285 

********* TESTING DONE *************
Right now in your constants file, this is gradient_descent.

You probably wanted L_BFGS. Also, you probably want something like:
```
if num_to_sample == 1
  gradient descent  # b/c we can compute the gradient here
else
  BFGS
```