\ccsdesc macro for ACM CCS 2012 classification

Macro for \ccsdesc snippets from http://dl.acm.org/ccs_flat.cfm

Example usage:
\classification{
   \ccsdesc[500]{Computer systems organization~n-tier architectures}
   \ccsdesc[300]{Information systems~Distributed storage}
}

Magic numbers are:
* 100 low
* 300 medium
* 500 high
Merge commit '31adbe5'