added a callback mechanism to support multi-project-aware plugins
renamed pmdcpd plugin to cpd in preparation for upcoming content refactoring
refactored cpd plugin to work with multi-project parallelized builds by limiting the plugin to only the root project since CPD is not thead-safe.
in addition, the pmd plugin is no longer applied by the cpd plugin, allowing it to be used independently.
Merge pull request #38 from mlueders/cpd_multiproject_support

Added multiproject support to cpd plugin