Fix for Bug #1234846

A function was registered with atexit which executed pynotify.uninit() without checking whether the user had pynotify installed or not.
Since pynotify.init() was called everytime a notification had to be sent, I've added uninit() in the same function
Fix for Bug #1264684 - Crashes when using GNotes and Tomboy synchronization
Fix for Bug #1253735

Also added a method 'get_browser' to PluginAPI so that the plugins which are written with GTG's browser in mind not get TaskEditor by mistake
Fix for Bug #1265349 - Error occurred while choosing Bugzilla plugin

Modified Bugzilla plugin to request a browser instance as Task Editor does not have any signal 'task-added-via-quick-add'
Requesting ui from PluginAPI sometimes returned TaskEditor and that caused the bug.
Since the signals connected to an object are removed when it is destroyed, I've removed the disconnect() call in the plugin
Fix for Bug #1265357 - Clicking "start task in hamster" crashes gtg

Now, the menu item "Plugins" > "Start task in hamster" will be disabled when no tasks or >1 tasks are selected.
Clicking it will change the label to "Stop Hamster Activity" when corresponding task is selected.