Checking for required external assemblies

This is a solution to issue https://github.com/TehGimp/KerbalMultiPlayer/issues/11 (which I had)

It checks for the 3 unity/ksp assemblies that are needed for the server and logs errors if they do not exist.

I'm presuming that this is a fair check as I doubt that windows is going to be finding these assemblies outside of the directory at run time.