CalcRegex Bugfix and Performance update for CssCompressor

* Bugfix CalcRegex :
** The greedy ".*" captured too many caracters until the last ")" of the file.
* Performance update :
** compile and store Regex that weren't.
** The regex to remove empty rules may be very time consuming, in my case it takes nearly 1minute for a 150kb css => Run it only if needed and compile it.