Added clustering to intelligently cluster date/time ranges
added s4 class definitions
Merge branch 'master' of https://github.com/ropensci/mdextract
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Fixed documentation.
Merge pull request #13 from lwinslow-usgs/master

New way to cluster date/time ranges added
merge conflict