Replace draw() function with display().

The main rendering function has been changed from draw to display. This
was done to differentiate between drawing methods/functions for objects
and the vroom rendering function. Objects which are rendered have draw
methods and there is a draw function which is used to batch render
collections of objects. This syntax is more consistent and hopefully
less confusing going forward.
Added Global.require decorator.

* The require decorator can be used to return from a function if the
  specified Global variable is not found.
Added draw function for batch rendering.

Objects can now be rendering using the following sytax:
   draw(func, args).for_each(points)
   draw(func, args).at(point)
Added support for object tracking.
Added object tracking.
Added support for object tracking.
Updated to use Buffer object instead of GLArray.
Updated __init__.py to include new functions.
Minor edits. Removed debug statements.
Merge branch 'master' of https://github.com/jvan/vroom