MDL-41057 csslib: CSS optimiser improvements.

This commit makes the following changes:
* Tidies up coding style.
* Tidies up phpdocs.
* Improve CSS optimiser unit tests to cover browser hacks.
* Improve backgound style consolidation.
* Improve border style consolidation.
* Fixed optimiser to handle browser hacks like *zoom.
* Improved @ rule unit tests.
* Fixed @rule breakages caused by OS specific rules.
* Added more unit tests to cover a bit more CSS3.

I am pretty sure this patch will now enable to optimsier to
work on bootstrap based themes should developers not wish
to compress thier compiled less CSS.