Add a pow dist save command

[mwatts15]

Saves the current data context and/or an optionally provided list of contexts currently within the local database (not necessarily related to the current data context) to an archive file

• Make the pow dist save take a working directory for files to package with
  the context (this is how sources like the LocalFileDataSource will have their data associated
  with them in a dist)

• Make the pow dist save pull the imports for a provided context from the current imports context, or an optionally provided single imports context. Only one imports directory is allowed since the dist represents a particular set of statements composed of a set of contexts.

• Have the dist make a structure akin to the current git directory structure (i.e., with an index and graphs directory) with the addition of a "manifest" parameterizing the dist

• Include a field for the imports context id in the dist manifest.

• Include in the dist a mapping of contexts to any file data associated with them (i.e., in a subdirectory), linked in a similar fashion to the way graphs are in the index

• Have the manifest specify a manifest version independent of the PyOpenWorm version for forwards/backwards compat

[mwatts15]

the dist concept has been absorbed into bundles. an explicit relationship between files and contexts beyond the grouping provided by bundles themselves has not been built into bundles.