Identified the cause of this bug (issue 131).

Need to fix, but might want to do some refactoring at the same time.
Fixed bug (131). Needs tests.
Started adding a test for timing functions in constructed Animations
Make test pass
Whitespace
Make tests pass after rebase
Store normalized timing in TimedItem._timing
so that we don't have to re-normalize when we compute durations.
Merge pull request #136 from rjwright/AnimConstructorEasing

Animation constructor easing