Changed browser_action and tabs events to use Streams.

Added code to browser_action and tabs to cleanup the state of the browser (at least when tests pass).

Added several more unit tests for browser_action and tabs.
Merge remote-tracking branch 'upstream/master'
Fix bugs in tabs.dart
Fix tabs.dart and test_tabs.dart
Merge pull request #60 from DrMarcII/master

Significant cleanup and improvements to tabs and browser_action