feat(pt): add ignore_time param for PeopleTracker

This feature allows the user to set the '$ignore_time' parameter in the
PeopleTracker per the People HTTP Specification of the Mixpanel REST
API. The user simply includes 'ignore_time' as True/False in their
properties dictionary and it gets converted to '$ignore_time' and
stripped from the properties dictionary. Included are tests and
documentation for this new functionality.
Merge pull request #27 from zpencerq/master

Add support for $ignore_time for PeopleTracker
Change MIXPANEL_TEST_ONLY to MIXPANEL_TEST_PRIORITY.

Mixpanel changed the meaning of the test flag in 2010, see
http://blog.mixpanel.com/2010/04/22/a-way-to-ease-your-integration-with-mixpanel-while-were-working/

It doesn't make sense to keep calling our setting MIXPANEL_TEST_ONLY, but
the flag might still be useful, so keep it around as MIXPANEL_TEST_PRIORITY
and update the various documentation bits.