New Timer utility for measuring CPU and wall times

Includes thread-aware timer registry for time
taking across multiple threads.
Modified tests to be more robust (esp. on OpenJDK6)

Unfortunately, time measurements are somewhat random
and cannot be predicted very well. The tests could still
fail randomly if some very unusual runtimes occur.
Completed JavaDoc comments
Merge pull request #7 from Wikidata/wdtk-util-timer

New Timer utility for measuring CPU and wall times