Created load and unload methods for style content and for game content allowing for simple override in both settings.
Unloading content in control manager also unloads control content, as per issue #8.
Added a contentLoaded field to the control collection and included load of content for all controls, for issue #7.
Controls can be removed from a control manager (unloading resources) as required by issue #8.
Renamed the methods for content load/unload, and changed the visibility of the local load to protected.
Implemented methods for disposing the resources used by a TextRenderer.
Added methods to simplify clipping out of bounds, this should help to resolve issue #6.
Clipping uses the newly added methods to ensure clipping is not done outside render target. Closes issue #6.