Improve rrd2whisper's handling of rrd files

Take into account the actual retentions in rrds and make whisper
archives similarly. Also allow selection of non-AVERAGE rrd archive
types
Added relatively comprehensive unit test for whisper.
Moved to class setUp and tearDown. Modified temporary file name to db.wsp.
Enforced file closing. This was necessary for better performance on Windows as Whisper was able to crash while still holding a lock on .wsp files. This behaviour did not occur in Linux.
Improved whisper test cases.