Minor improvements

Hi. First of all, I really like the glbinding code. It's sleek. I have found three small things I would like to change though. These are:

* although the default underlying type of scoped enums is int, `CallbackMas` was casted everywhere to an unsigned int. i changed that and also reduced the size to an unsigned char
* `AbstractFunction` had a virtual destructor, which is unnecessary. I removed it so that 2787 Function objects need less memory ;)
* callbacks were passed by value rather than by const ref.
this needs to remain virtual