Adding support for Prismatic Joints

CCSlideJoint is an Objective-C wrapper (as part of CCBox2D) that adds
support for Prismatic Joints in Box2D.  A Prismatic Joint is a joint
that is fixed along one axis (for example, an object an "rotate" along a
Y axis only while having a fixed X, or vice versa).  This is especially
useful in vehicle objects where you have 2 turning wheels and 2 fixed
wheels - The prismatic joint would be the fixed wheels.
Initial Commit

Replaced CCSlideJoint with CCSliderJoint
Filled in support for Prismatic Joint
Make nodes subclasses of CCNode instead of CCSprite.
Revert "Make nodes subclasses of CCNode instead of CCSprite."

This reverts commit 71046599d9d5949e12f97568b880f30bfc3b9e91.
Merge branch 'feature/node-based' into develop
Merge branch 'master' of github.com:bgulanowski/CCBox2D into develop
Remove some redundant code.

I wanted to conditionally convert the position, but the conditional logic required is more complex than just converting all the time, even if it's not necessary. Fortunately, the time's it isn't necessary are also the time's when the node's distance from the root is small and the transform calculations should be fast if necessary at all.
Merge commit 'b8c52fcf16c66d4bf2a2928ecf0ae4cfb07d5c6b' into develop

Conflicts:
	.gitignore
Add new joint class. Keep C++ members completely private.
Break out shape and fixture handling into a new class.

Also, no replace a whole suite of ivars with a single b2BodyDef ivar, which is deleted when the body gets created, and recreated when the body is destroyed. We either have a body or a body def, never both.
Replace collision methods with collisions blocks. Fix bugs.

Wrong property generation declaration for world. Wrong array when iterating shapes.
Add #import.
Update to block-based collision updating.
Fix memory problems.

Shapes have to be allocated so that they're kept around. This differs from the normal usage pattern of creating a body def and immediately making a body. Instead we're keeping the body def around, and need the shape to stay around, too.
Fixes for incomplete property definition, fixture positioning, and block invocation when block is nil.
Shapes are NOT offset relative to a body's global position.
Fix a couple of leaks.
More accurate property types. Access to shapes data.
Disable warning that bugs me.