Fixed build error and null reference exception

Jon,

Just a couple of small changes:

- I know you're aware of the build error (issue 21). Removed reference to "src/ZXing.Net/common/Collections.cs" to build. 
- The other commit is related to an exception being thrown in "/src/ZXing.Net.Mobile/MonoTouch/ZxingSurfaceView.cs". Just added a null check to stop the exception. I noticed the exception while troubleshooting another issue (might be in touch about that soon).

This is my first pull request - please let me know if anything else is required.

Cheers, 
Ben