Implemented data blobs for device buffer updates

I know this is ugly. It does boxing of arrays and forces some vtable calls. Fortunately, updating a buffer is not a frequent operation. I simply don't know the better way to do it with generic vectors and structs... It is mostly encapsulated in the gfx-rs though, so we are free to re-implement it if we find better ways.
 
Closes #82
I thought you didn't need the cast here... ie. it would implicitly convert?