Bugfix ply support

(Copied from #71)

This commit adds support for the `int8`, `uint8`, etc. property type names
in addition to the `char`, `uchar`, etc. that were supported before.

I don't know when or why people started using the alternate names with the bit-size suffixes, but now there are lots of PLY files out there that use them.  They seem not to be part of the original PLY spec, but I think they're considered to be now.  (In fact, my [python-plyfile](https://github.com/dranjan/python-plyfile) package used to output them as of one day ago.)