Bugfix [81403440]. Rename www.buyusainfo.net tp mr.export.gov
Merge pull request #34 from GovWizely/james/mr-export-gov

Developers see new URL for MRL info [81403440]
Oversight in '#parse_american_date' describe title
Recreate CSL indices before importer specs run.
String#typeize now returns a symbol.
Makes model_class an instance method of Importer.
Importer#import_then_purge_old.
Add purge_old to CSL;

Also: add spec examples that test whether or not an importer can purge
old documents.
Adds can-purge-old-documents to Parature FAQ.