Code cleanup + Vector Icons

#### COMPLETE ####
* Made some very minor changes (fixed small class-name typo)
* Removed raster icons and replaced with vector icons from [Modern UI Icons](http://www.modernuiicons.com)
* Exposed IconData property on the `Notification` class
* Updated demo to reflect changes

#### TODO ####
* Need to try and figure out how to make more compatible with MVVM (creating `GrowlNotification` in MainWindow code-behind is not ideal).
* Would like to split into separate library/demo projects