improve code quality of bridgedescs module

The code quality of bridgedescs module should be improved using the reports generated by Findbugs and PMD.

Intended results:

• increase test coverage of org.torproject.collector.bridgedescs to something reasonably close to 100%
• add ant tasks for both
• define the set of rules that will be used for all java projects
• improve code quality of the bridgedescs module based on the reports