Support for registering custom observers, better error logging, view_thread bug fix

- Check for nH class to identify view_thread observer (instead of Bs) -
fixes bug where this observer in some gmail configurations fails to fire
(fixes #115, #116).
- More correct error logging using console.error
- Move dom observer configuration into a separate initialize function
for code cleanliness
- New api.observe.register method which allows custom DOM observers to
be registered. This means people can have their own application specific
DOM observers without Gmail.js having to support every event/action
under the sun. (closes #113)