File Associations (Apple)

Closes #665 by adding working file association support for Apple.
Changes include:
- Added new ${VARS} to CMakeList.txt:
   - site URL
   - project icon (mmpz)
   - mimetype prefix identifier (net.sourceforge.lmms)
- Rewrite of Info.plist file based on VLC and KeyPassX.

![image](https://cloud.githubusercontent.com/assets/6345473/2911395/92cda9bc-d65a-11e3-9cf0-2d1dfebbea77.png)