Update NUnit to 2.6.1

This updates the NUnit libraries to 2.6.1. 

I originally tried updating to 2.6.2 but there is a bug in that version that is preventing testing on Mono. ([NUnit bug report](https://bugs.launchpad.net/nunitv2/+bug/1079296)) was filed and it looks like the fix will go in the 2.6.3 release.  Once that is released, I'll test it out.

This has been tested on Win7 .net 2 and 4 and Mac OS X Mono 3.0.10.