Box2D library added.
Added angle conversion.  Updated transform for component-entity system.
Merge branch 'collision' of https://github.com/bcrist/PBJgame into collision
Definition and implementation of rigidbody class.  Included commented-out
methods for use when implement an Entity class.  Commenting minimal.
TextureFont loading, UILabel
UILabel done
frame interval benchmark
Mesh, parts of UIButton
UIButton stuff
UIButton done (mouse/keyboard untested)
UIContainer/UIPanel/UIRoot work
batcher
Starting networking
ui parenting problem sol. started
more net
ui input handling, uiroot/uipanel

UIContainer functionality moved to UIPanel
batcher stuff
updated rendering to use batcher

bug w/ texturefonttext
fixed texturefonttext batcher rendering bug
InputController/Window callback improvements

Fixed listener cancellation in Window
Added listener cancellation to InputController.
Fixed InputController::destroy()