added COBYLA (no REST interface yet)

********* PEOPLE *************
Primary reviewer: @sc932 

Reviewers: @suntzu86 

********* DESCRIPTION **************
Branch Name: denizokt_370_constraints
Ticket(s)/Issue(s): Closes #370 

********* TESTING DONE *************
make test
make style-test
this docstring should be more precise: this fcn calls get_scipy_optimizer

it'd also be nice to be explicit about the return:
```
:return: blah
:rtype: blah with shape (foo, bar)
```