Enable dumping design docs to files

Dump the design docs so we can copy them to the platform and apply them on
a couch deploy.

`rake couchrest:dump` will do the trick.

This is similar to the migrations but instead of uploading the design
documents to couch it stores them in
  tmp/database/design.json
within the rails directory.

database is the supposed database name without prefixes or suffixes
design is the name of the design doc CouchRest model would have created

The files also contain a couchrest checksum so couchrest can detect they
are up to date.