draw_elements: Separate count from the (optional) element list. Allows buffered drawing.

Fixes #13
Expose glBindAttribLocation.
Merge pull request #16 from robn/draw-elements

draw_elements: Separate count from the (optional) element list. Allows buffered drawing.
Merge pull request #19 from metajack/bind-attrib-loc

Expose glBindAttribLocation.
Merge git://github.com/mozilla-servo/rust-opengles