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 name is kind of confusing... this fcn actually performs the optimization.

also this method should be private (leading underscore).  I'd make a note in the docstring that users shouldn't call this method directly but use optimize() instead.