API improvements and simplifications

Here are a collection of API improvements/simplifications.  In summary, the following things were changed:

* The .enable()/disable() methods were removed and are now automatically called once you set a uniform/buffer.  You can still do the same thing by calling the normal WebGL methods if you want, so no real features were lost.
* The extra .set() method was removed, since you can do the same thing by just assigning to the attribute with the property syntax.

@chrisdickinson, @hughsk, @gre: As the main users of this library, what do you guys think about these changes?