Move NUnit dependency and invoke nuget always in cmd script

I noticed that the `build.sh` file has a hard-coded version of FAKE. This does not sound right to me.. or is there any reason for that?

Also, `build.cmd` always triggers `nuget install`, which will automatically update FAKE if you have an old version. It sounds sensible to do this on *nix too.

@caindy - Could you please check that this works well on a Mac? (Or in case I'm missing something, let me know what the problem with latest FAKE was...)