Add support for $ignore_time for PeopleTracker

This feature allows the user to set the '$ignore_time' parameter in the
PeopleTracker per the [People HTTP Specification of the Mixpanel REST
API](https://mixpanel.com/docs/people-analytics/people-http-specification-insert-data). The user simply includes 'ignore_time' as True/False in their
properties dictionary and it gets converted to '$ignore_time' and
stripped from the properties dictionary. Included is a test and
documentation for this new functionality.


I wasn't sure if the user should use `ignore_time` or `$ignore_time` in their properties dictionary, so I just erred on the side of consistency and did it without the `$`.

Also, the Django 1.3 tests failed for me when running tox but they failed before the changes anyways.