Fix fromCssValue() methods to return undefined if the input value is invalid

Currently, the various fromCssValue() methods handle bad input differently.
This change standardises the handling so that missing logic is more obvious.

It also adds a console.assert() to the main fromCssValue() function to detect
this condition, and a TODO to fix the keyframe normalization process to ensure
the assert is never hit.
Clean up the fooType classes used for adding and interpolating property values

- Fix some fromCssValue() and interpolate() methods to allow a tests to be
  added for various types of composition.
- Add zero() methods where required and remove some that are superfluous.
- Add TODOs for cases that aren't currently handled.
- Require the arguments to interpolate() to always be provided.
Add more invalid input checks
Merge pull request #1 from alancutter/cleanUpFooTypes

Add more invalid input checks
Merge pull request #345 from steveblock/cleanUpFooTypes

Clean up the fooType classes used for adding and interpolating property values