icon engine plugin for font awesome

Added icon engine plugin which provides icons from the Font Awesome (http://fontawesome.io/)
Add a few icons to the main menu
Several new actions in edit menu

* add several new actions to edit menu: Cut, Copy, Paste, Format Strong, Format Emphasize
* new class MarkdownManipulator which is responsible for manipulating the markdown text document
Merge branch 'feature/font-awesome' into develop