Fixes #826

Added helper methods to `RajawaliRenderer` and `RajawaliScene` for
`Plugin` addition/removal.

Signed-off-by: Jared Woolston <jwoolston@tenkiv.com>
Merge remote-tracking branch 'origin/master' into scenegraph

Conflicts:
	src/rajawali/scene/RajawaliScene.java

Signed-off-by: Jared Woolston <jwoolston@tenkiv.com>