Add config helper method get_string

The normal `adhocracy.config.get` method returns unicode objects in
contrast to `pylons.config.get` which returns strings.

If string objects are required explicitly, get_string can now be used.
Use get_string to create connection to solr

If a unicode string is passed to solr, this makes httplib fail at some
point, where message_body (containing a utf-8 encoded string) is added
to msg (which was then a unicode object).

This is fixed by making sure the solr connection url is a string, not a
unicode object.