Allow wildcards for files in the NUnit test runner task

For each assembly to be tested, you have to explicitly name it in the NUnit task configuration or the .nunit file.
We would like to use a wildcard pattern (e.g. *.Test.dll) to select files for unit testing, rather than having to edit our task or .nunit file every time we add tests.

For reference, see the TeamCity implementation here: http://confluence.jetbrains.com/display/TCD8/NUnit