Added binary ply exporter.

I added a binary .ply exporter (identifier: "plyb"), implementation is very similar to the ascii-based one.
Hmm, I know you're following what was done for the ascii format, but I'm a little confused after reading the code. We only write the header configuration for normals and other properties (e.g., "property float nx") when they exist. Yet here we write out the values? Can't we simply skip them? Wondering why the ascii exporter doesn't do that...