Add overload Crop method to accept Rectangle (as well as coordinates)

I have built upon @ne4ta's PR:https://github.com/benfoster/ImageResizer.FluentExtensions/pull/4 and added an overloaded Crop method to accept a Rectangle as well as the raw x/y coordinates.

I have VS 2013, so I have been forced to upgrade the solution to VS 2013, but it should be backwards compatible up to VS 2010 SP1 (http://msdn.microsoft.com/en-us/library/vstudio/hh266747%28v=vs.120%29.aspx).

I have also added some tests to test the new Crop methods, but I am not able to run the tests as I don't have an MSpec test runner, but my own personal tests work fine and trust the MSpec tests will pass also.