CSS rewriters don't handle a CSS file having a BOM

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If a CSS file has a BOM (Byte Order Mark) it was causing problems with some CSS 
rewriters, in particular the combiner because the resulting combined CSS would 
have a BOM embdeded in it rather than at the start.

This has been fixed to strip the BOM from CSS, reinstate it a BOM at the start 
of the combined CSS, and take the BOM into account when deciding whether CSS 
files should be combined (we now only combined CSS files with the same charset).

Original issue reported on code.google.com by matterb...@google.com on 23 May 2012 at 1:10