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

This will export catalogs from the ActiveRecord storeconfigs database to a tarball for import to PuppetDB through an as-yet unfinished import command. This is intended to aid in migration of new users to PuppetDB, helping to prevent failures or unwanted changes due to suddenly missing exported resources, not as a completely faithful reproduction of the original data. Thus, only exported resources are included, and no catalog is produced for nodes without any exported resources. After every agent has run once, this data will be overwritten with the current, correct data anyway.

This pull request includes the start of an acceptance test (in `acceptance/pending`), which can't be finished until the import command is written.