Adds the Mixpanel People API

https://mixpanel.com/docs/people-analytics/people-http-specification-insert-data

Note: The tests are fragile. The existing test pattern was followed, but
it is dependent on the ordering of dictionary keys which are arbitrary.
Removes print statements
Added Anton and Solomon to AUTHORS. Thanks for the pull requests!
Merge branch 'features/people-api' of git://github.com/lincolnloop/mixpanel-celery into lincolnloop-people-api

Conflicts:
	mixpanel/tasks.py
	mixpanel/tests/test_tasks.py
	setup.py
Tweaked the tests so that the failure recovery tests work.

Changing module-level global settings is kind of icky.
Added Peter Baumgartner to AUTHORS. Thanks Peter!