First try for robust get_all

json requests simply get a 400, html requests will get a flash warning
and their query will be discarded.
Probably the URL could/should be rewritten, too.

See TurboGears/tgext.crud#12 for discussion.
Returning nothing is better... TurboGears/tgext.crud#12
Merge pull request #15 from moschlar/develop

Fix for #12 - Robustness of search field