Improve interface for buffer binding

[abhimadan]

#35 adds an initial pass at this. It makes it easy for users to call, because you bind one attribute at a time. However, this prevents attribute buffers from being interleaved, which improves performance in GPU programs. The problem with this version, however, is that it's restrictive for users, since they need to construct the interleaved buffer before passing it to Calder (which doesn't make it much easier than using raw WebGL).

Hopefully, we can find a good balance between performance and ease-of-use.