Implement stage.lockOrientation() in HTML.

Not implemented in most browsers, but coming soon to Chrome and IE
mobile. Note that you have to be in fullscreen first.

Also should work in Firefox, but fullscreen on mobile is awkward:
https://bugzilla.mozilla.org/show_bug.cgi?id=779324

Closes #4.
Add System.renderer.

Exposes a low-level createTextureFromImage method for creating Flambe
textures from native image data.

Breaking change: System.createTexture and hasGPU have been moved to
System.renderer.