Content now building in dummy XNA project

And being copied over to MonoGame. Used a post-build event for this, which took
some reading around to find. Initially also tried to faff with getting the
content pipelines to work with the MonoContent project, but these don't seem to
ship with 3.0.1, and the source has too many dependencies for me to be arsed
with it. This is way simpler and works for Windows at least.