Adds support for nCrunch.

Adds necessary files and ignores so that nCrunch can automatically run the tests.

Note: The way the tests are setup right now are very brittle when run in parallel.  Just looking at the test failures, it appears to be a problem with file locking and multiple tests manipulating the same on-disk resources.  I would encourage resolving this (though I can appreciate that it is low priority).  The first thing I would try is to have each test make a copy of the resource it wants to manipulate, work with it, then delete its copy when it is done.  This would allow several tests to simultaneously manipulate the same underlying set of files without conflict.
Behind the scenes, NCrunch builds everything out into separate folders and the runner has a custom assembly resolver for locating assemblies on disk.  It does not use the CLR's built in assembly resolver.  After getting all other tests passing, this was the only one that wasn't passing.  The name of this test suggests to me that it is testing the behavior of LibGit2Sharp under a very specific circumstances (shadow copying by the CLR).  I assumed (based on the test name) that it was unlikely to work correctly under NCrunch due to the way NCrunch does assembly resolution so I ignored it.

It may be possible to get the test running under NCrunch, but my thought was that it probably wasn't worth the time investment required for a single test.

*Personally* I am pretty loose with NCrunch test ignores.  Whatever build server I have usually runs all of the tests anyway, NCrunch is there as a convenience for getting a tighter feedback loop.  If one test is missing from NCrunch runs it isn't a big deal to me.