Added momentum scrolling and "rubber band" snap-back code from Travis Fischer.

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branch : momentumScrolling
Added documentation and private properties for all iVars. Cleaned up the code to conform with Core Plot coding guidelines.

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branch : momentumScrolling
Added utility functions to compute the minimum and maximum of two NSDecimal values.

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branch : momentumScrolling
Added an absolute value function to the NSDecimal utilities.

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branch : momentumScrolling
Updated the momentum scrolling and elastic plot range code to use CPTAnimation instead of blocks. Added support for the y-range in addition to the x-range.

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branch : momentumScrolling
Updated the Mac CPTTestApp to demonstrate momentum scrolling and elastic plot ranges.

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branch : momentumScrolling
Fixed a crashing issue when using attributed strings under iOS 5 and earlier.
Updated the unit tests to match the updated pixel alignment code.
Force plot symbols to redraw when switching between Retina and non-Retina displays.
Merged the momentumScrolling branch into default. Fixed issue 531.
Fixer a bug with elastic plot ranges.
Built new HTML docs.