Add a `puppet storeconfigs export` command to dump the database

This command can be used to export a copy of the ActiveRecord
storeconfigs database (only exported resources, and only nodes with
exported resources) which can be imported into PuppetDB.  This is useful
when migrating from old-school storeconfigs to PuppetDB, as it allows
agents to run without losing any of their exported resources for a time.
Once every agent has run once, the database should be up-to-date in any
case.
Don't export catalog environment

This isn't actually a legal key on the catalog, so exclude it.
Write the storeconfigs database dump as a .tar.gz

Previously this was a single file containing many JSON objects, for the
catalogs. Now it's a .tar.gz (or .tar if gzip can't be found [or
directory if tar can't be found]) containing one file per node. The
tarball also includes a metadata file specifying the nodes which are
present, the timestamp of the export, and the version of the "replace
catalog" command which ought to be used to submit the catalogs to
PuppetDB.
Add a pending acceptance test for storeconfigs export

This test can't be finished until the import functionality for PuppetDB
proper is complete, but it seems better to have this start than nothing.
Conform to the PuppetDB export format
Document the `puppet storeconfigs export` command

This also forms the beginnings of a migration guide.
Merge pull request #438 from nicklewis/storeconfigs-migration

Add a `puppet storeconfigs export` command to dump the database