Mesh attribute binding fix and the indexed draw function

Should be accepted after #33 (includes those changes).

Fixes #31 and #32.

Any suggestions on how to make indexes more usable are always welcome ;)
I noticed this in a few places. I admit that `std::mem::size_of::<u16>()` is pretty wordy (not even sure I got that right). But maybe we can use a utility function or macro to self document where the 2 came from.