Content Pipleine Xamarin Studio Addin

This PR includes

1) An Addin for Windows and Mac for Xamarin Studio to compile the following content
  a) Textures  - DXT and PVRTC compression support
  b) SpriteFonts - Can use system fonts like XNA does, BUT you can also put the font file in the same directory as the .spritefont and it will pick that file up. 
  
It also has some changes to support Audio processing on Mac and a fix for NAudio on Windows when producing .mp4 files. The Audio processing seems to be working ok on Mac and can convert a .wav to an .mp4, and a .wav to an .xnb which can then be used. 

This has been tested on Mac and Windows.