added ofxBulletStaticPlane for creating floors, ceilings etc.

hi Nick,
you've done a great job on this addon.
ive forked your repo and have been adding some stuff that i've used in the past on my own bullet projects.
i want to keep my fork as close to yours as possible and will be sending a lot of my PR updates.
hopefully we can work on this together.

this PR is adding ofxBulletStaticPlane,
which is used for creating floor or ceiling planes which are infinite.

one thing i needed to do is create a `removeShape()` method in ofxBulletBaseShape to isolate the shape destruction code so it can be overridden by ofxBulletStaticPlane. this is because the `_shape` needs to be cast back to `(btStaticPlaneShape *)_shape` when deleting otherwise it will cause leaks.

let me know what you think.

L.