feat #739 extract css.Units util

Extract existing unit manipulation tool.

Add some new methods for reverse direction calculations and also some
helper methods needed for IE8- polyfill of
`getComputedStyle(width|height)`.
They're all in this class instead of `utils.Dom` to avoid circular dependencies.

Also fix some small issues:
- `font-size` -> `fontSize` in a check
- `fontSize` returned can be non-pixel in IE; taken into consideration now
fix #739 consider borders/paddings in getStyle(width|height) in IE