Add Blacklight.secret_key for encrypting and signing Blacklight messages

- under Rails 4, this defaults to the secret_key_base and requires no
  additional action.

- under Rails 3, this must be explicitly set. An example initializer is
  provided.
Add Document#exports_as? for checking if a document has the given export format
Update bookmarks to support the full range of index export formats and
fix refworks and endnote export (provided by blacklight-marc)

Also, add a generic way to work with the refworks callback API
Merge pull request #876 from projectblacklight/bookmarks_export_without_marc

Bookmarks export without marc