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). The image below shows output in Xamarin Studio when running the sample project. The default view is currently open. A new error is displayed about once every second or so.

![Screen Shot 2013-03-18 at 4 01 09 PM](https://f.cloud.github.com/assets/18481/272940/72d57d4e-9013-11e2-9f17-7c9b3cd6176b.png)


This is my first pull request - please let me know if anything else is required.

Cheers, 
Ben