Merge slackhappy/whisper with modifications

Closes pull request #10

Enforce merges to be strictly between two whisper files with the same
archive configuration and do a direct archive copy. This solves the
issue of time changing between archive copies.
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
Initial package