mixpanel: correctly serialize lists in track properties

Blindly forcing tracked property values through port.to_u was a bad
idea. According to Mixpanel docs, properties can be strings, booleans
or lists.

Report and fixed by Hector Garcia, although I used a different patch.

Fixes #45.
mixpanel/tests: fix tests that depended on dict iteration order
gitignore: ignore trial's temporary directory