Go mono!

This pull request essentially enables _System.IO.Abstractions_ to be used on Mono. In particular, it fixes #82 as well.

The trick which got me into fixing it actually is to remove the recursive access for `MockFileSystem.Directory` (see 3e17340). The rest was to fix all backwhacked tests to run as well on slashdotted systems. I followed a lazy strategy by introducing a `MockUnixSupport` helper. I aliased it in tests with `XFS` for the sake of brevity.

**All tests pass both on Mono and Windows.**

I tested on Win7 w/ Net4 and Linux 3.16.3-1 w/ Mono3.4.

@nosami @tathamoddie It was sort of a stretch to get this running on Mono as well, but I suppose it's worth the effort. Feedback and comments welcome.