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

```MOAIAnimCurve``` doesn't have many ease functions.  In fact, they're almost all just variations on ```powf(t, rate)```.  That sucks.  :spaghetti: 

We could just add our own ease types in C++, but that's not very flexible.  So here's ```MOAIAnimCurveCustom```.  It's just like ```MOAIAnimCurve```, but it allows you to define a lua function (using ```curve:setCallback(easeFunction)```) that will be used for all interpolation by the curve.  :curly_loop: 

This will let us define a library of lua eases to use with ```ease()``` in the ActionHelpers.  :smoking: 

I haven't noticed any slowdown compared to using regular ```MOAIAnimCurve```, even with a bunch of eases running simultaneously, but that's something that we should be watching out for if we start having performance problems.  :bread: 

I should note that this is all based off my brother's work: https://github.com/IkeBart/moai-dev/commit/2e3071931da3e9ac6460645d60311857be150738

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The one thing that isn't done here has to do with the fact that I don't remember C++.  I'm getting a compiler warning because apparently I don't know how to do inheritance right.

MOAIAnimCurve.h:46: 'virtual void MOAIAnimCurve::GetValue(MOAIAttrOp&, const MOAIAnimKeySpan&)' was hidden
MOAIAnimCurveCustom.h:29: By 'virtual float MOAIAnimCurveCustom::GetValue(const MOAIAnimKeySpan&)'

I want ```MOAIAnimCurveCustom``` to be just like ```MOAIAnimCurve```, except that when it calls ```GetValue(const MOAIAnimKeySpan&)```, I want it to use its own method.  That's all.  Can anyone tell me what I'm doing wrong? :question: 

Also, I had to remove a lot of ```const``` declarations on functions all over ```MOAIAnimCurveBase``` and its subclasses in order to get ```GetValue(const MOAIAnimKeySpan&)``` to reference its own ```mCallback```.  Does anyone know of a better way to do this?  :question: