Append frames to animation

Fixes #943

**Breaking changes**: corrected spelling of "Indices" in several spots. The change breaks `addByIndicies` and `addByStringIndicies` but those can be easily fixed with a find and replace.

Added `append` methods as counterparts to the existing `add` methods in order to add to existing animations.  `FrameRate` and `Looped` parameters were left out since adding to the animation should not change those behaviors.

Fixed `addByPrefix` improperly sorting when no postFix (".gif", ".png", etc) exists.