Add enum type property to XGSMarkdownTag
Create Icon buttons for tags on keyboard accessory view

Add a new button class XGSIconCircularButton that has the following specificities:
- uses a CAShapeLayer as backing layer and constraint the shape to always be a circle using the MIN(height,width)
- the circle has a 1 point outline, the colour is controlled using the tintColor property of the button.
- the font of the titleLabel property of the button is set to the FontAwesome font (http://fortawesome.github.io/Font-Awesome/).

The markdown buttons in the keyboard accessory view are now instances of XGSIconCircularButton.