[super] Functionality for working with data in "source" form

[rufuspollock]

In terms of analogies: currently dpm is more like pip / npm / gem / apit-get. This functionality is more like git / hg / svn but for data.

• init (already existed) - Rename create command to init #10
• clone / checkout (rename of download)
• add (remove) and push - add command to add resources to push list #12
• pull - push command to push material to remote CKAN #11

For general overview and details see: https://github.com/okfn/dpm/blob/master/doc/new-plan-2011-nov.rst