update setup.py for OSX and add colander warning

********* PEOPLE *************
Primary reviewer: @sc932 

Reviewers:

********* DESCRIPTION **************
Branch Name: eliu_colander_warning_and_setup_py_cleanup
Ticket(s)/Issue(s): #412 #413

1) update setup.py to be able to find python in OSX
2) add warning about colander version mismatch

********* TESTING DONE *************
make test