Theme js

This is a big one, it creates the global Messenger object, creates a way for themes to add their own js, and renames some internal classes to be more descriptive.

For more, see the new [changelog](https://github.com/HubSpot/messenger/blob/d252476a499a533010442bcbab31627a982b4ca6/CHANGELOG.md)

Thoughts @TrevorBurnham @bosgood?