Merge pull request #1 from christopher-mccreadie/SoTLevelEditor

SoT level editor merged
Create README.md
Made the ContentManager factory generic and extensible. Also modified existing code to use the new interface.
ContentManager is now used in place of 'new Texture2D(*)'
Put new content manager into the ContentViewer as well. Is there some way to share this file across all projects in the solution?
Added Default Floor plan image
Changed New Level text to default to 'NewL  Level'