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
currently all the implemented domains have a valid get_constraint_list function. so i added it to the domain interface. if we implement a domain that does not have one, we can throw an exception from within the method.