Added support for collection observation.

Collections can now be observerd with callbacks by calling
client.observe(collectionName,...). This returns an
observer object that can have methods attached to it for
additions, removals and updates. Those callbacks are called
immediately on DDP messages. The observation can be stopped
with a call to stop().
Bumped version due to API change.
Added tests for collection observation.

Observations now receive cleared fields and old field values (on change).

Some linting.
Merge pull request #45 from kubat/master

Added support for collection observation.