added reverse playback and "bounce" looping (back and forth)

added functionality to `MovieClip` to allow reverse playback and "bounce" loop -- play forward and backward as a loop.

both are fully compatible with `MovieClip`'s existing features. setting `bounceLoop = true` will set `loop = false` and setting `loop = true` will set `bounceLoop = false`, since the two cannot both be true at once.  `reverse` works whether or not `loop` or `bounceLoop` are set.

there are two commits in this pull request because i implemented these changes against an out-of-date version of `MovieClip` and had to go back and reimplement them against the latest version.  better to look at the MovieClip.as diff than to look through the individual commits.

i haven't tested with your unit tests as i don't have that test suite set up, but i manually tested all the permutations of `loop`, `bounceLoop`, `reverse`, and an `Event.COMPLETE` handler that i could come up with, and have been running with this code successfully for a couple of months now.