Created MOAIAnimCurveCustom.  It *should* be able to register a lua callback... but that's it for now.
So I think this is ready, but it doesn't compile.  I'm pretty sure the reason why is just that MOAIAnimCurveCustom is trying to reference 'non-const' things in a 'const' method.  I'm going to remove the 'const's next, but it seems like there should be a better way to do that.  Anyway, I'm committing now in case I want to go back to a time when 'const' still existed.  Oh, C++.
Basically removed all instances of 'const' in all of Moai ever.  This is probably the wrong thing to do, but I don't know how else to get it to compile.
So making things virtual makes it so you can inherit them?  But I'm still getting warnings that I'm hiding seemingly unrelated functions.  I don't get C++.
Merge pull request #20 from mindsnacks/feature/anim-curve-custom

[WIP, but there's just one thing] - MOAIAnimCurveCustom