Breaking css minification bug fix: condense_hex_colors breaks 8-digit hex strings

[StefanTodoran]

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Changed the regular expression in condense_hex_colors to match only 6 digit hex strings using negative look-ahead. Previous behavior would break 8 digit long hex strings, because it would result in an invalid 5 digit hex string. Also added a test file for CSS minification, which contains some tests for hex color minification.

Closed the original request and reopened this one since I had made some modifications to the master branch which I didn't intend to include in the pull request, now the changes are in their own branch.