Allow for mounting as an engine

Sort URL generation should be irrespective of the app/engine url_helpers to allow for mounting as an engine- #url_to replaced with #resource_path

fixes #5, fixes #8, fixes #13
Merge branch 'master' of github.com:bespokepost/activeadmin-sortable into bespokepost-master
release 0.0.2
Generate correct sort URL if query parameters are present
Merge pull request #18 from schuetzm/fix-query-params

Generate correct sort URL if query parameters are present
release 0.0.3
use auto_url_for to get url of resource

resource_path get the current resource path (doesn't use the parameter provided)