Added a crude Test Suite using unittest.
Updated the README accordingly.
Added tests for Tweenable interface.
Added the Tweenable interface.
Implemented the Tweenable interface.
Tests pass in Dartium, but dart2js still refuses to compile.
Better Test section in the README
Loosen `_target`'s type from Object to var.
Fixed typo in filename tween_accessor.dart
Misc cleanup