AutoFixture.NUnit

This adds a library for using AutoFixture with NUnit that is a modified version of the AutoFixture.xUnit.net library.  We created this for internal use at my company, and I would love to contribute this to the AutoFixture project if you think it would make a good addition.

Some slight ugliness because of NUnit's lack of xUnit.net's customization: 
* AutoTestCase attributes must be provided with the type of the class and the name of the method they are applied to, for reflection purposes.
* NUnit reflects over the TestCase attribute many times when running tests.  To prevent AutoFixture from having to regenerate the values needed for a test multiple times, the results are stored in a static ConcurrentDictionary after the first time.