Implement ImageSize on gfx::TextureHandle

[bvssvni]

See #24

It might be possible to hack around this by:

1. Add a Size property to this crate, use piston-quack to implement it for gfx::TextureHandle
2. Add an auto impl with a where clause for ImageSize using the Size property.

If this works, then the compiles should allow 1) because the tuple (Size, gfx::TextureHandle) is owned by this crate, and 2) because where clauses can be used as refinements of impls.

[bvssvni]

It might be this doesn't work because ImageSize and T is not owned by this crate, and refinements are ignored.

[bvssvni]

Confirmed. This does not work.

[kvark]

Perhaps, this is for the better. We could return more info from image loading than just a texture handle. For example, a flag saying that the image had alpha in it. Or the number of bits per component.

Edit: actually, all this info is available through gfx::TextureHandle, so it would be nice to return it, after all...

[bvssvni]

@kvark There is one alternative, where Gfx depends on piston-texture and implements ImageSize. https://github.com/PistonDevelopers/texture Not sure if that's the right thing, though.