Some Mac-build cleanup

I'm starting development on a few small tweaks myself, and I found more stuff than I wanted trying to get in the repository.  So I got them ignored, and in the process found that there was cruft - which I cleaned out.  As noted in e01d556 the cruft comes from the Oculus SDK, so it's up to you whether you want to accept those removals.  The ignore file mods should prevent those from sneaking back in if you do.

I've also included a modification to the readme based on the lack of the assets folder.  The lack of this folder causes a build failure on MacOS.  Simply adding the folder allows the build to finish, and then the lack of contents simply results in a runtime "file load failed" error in the Tuscany (Oculus World) demo.  Copying the assets from the original SDK then makes everything happy.