Unbundled all the things
    
    Unbundled a bunch of libraries (irrxml, poly2tri, unzip, minzip)
    Added a wrapper for fast_atof.h.  The irrxml upstream took almost all of
    the assimp project's changes, but deleted a few in the latest release.
    This wrapper falls back to the irrXML functions where possible, and
    otherwise used assimp's functions.