Switch unit tests to NUnit

Hi Andrew,

I have previously converted the unit tests in *AForge.NET* to *NUnit*, since I find the *NUnit* test framework more accessible than *MbUnit*/*Gallio*.

For convenience, I thought I should provide a pull request with these updates. (The pull request also contains a default *.gitignore* file which should facilitate version control.) 

If you are interested in switching the *AForge.NET* unit tests to *NUnit*, be my guest and pull this request. If not, just leave the pull request without notice.

Best regards,
Anders @ Cureos