GET requests on resource should be able to fetch just timestamp data

[jrwdunham]

That is, certain params should cause GET /forms to return an array of small objects containing only, for example, datetime_entered and datetime_modified attributes.

[jrwdunham]

Useful for data visualization as well as for client-side cacheing with periodic checks on the server for updates. No need to ask the server to serialize and send a bunch of stuff we may not want.