let length interpolation be independent of the expected length types
let length addition be independent of the expected length types
remove explicit initialization for zero point
Remove explicit initialization from toCssValue and fix bug in default interpolation case.
replace 'percent' label with '%' in length objects.
replaced length toCssValue with generic version
Make fromCssValue generic.
review changes
best test evar
fix indentation and test.
Merge pull request #47 from shans/moreLengthSupport

More length support