Fixes #215 - Add support for simple compression 

Here is a patch that removes some whitespaces and all comments when Configuration#isCompressing() returns true.

The compressed CSS still preserves the new lines because otherwise I'm not sure how the source maps support would work.

I've added the checks `isCompressing()` directly in CssPrinter (vs. ExtendedStringBuilder) because this way I have much finer control when to ensure a separator/new line and when not.