signature address parsing improvements and tests

Fix sigatures not supporting [10.0.0.0/24, !10.1.1.1] notation when
used directly in a rule instead of through a variable.

Add tests for Bugs #815 and #920.

https://redmine.openinfosecfoundation.org/issues/815
https://redmine.openinfosecfoundation.org/issues/920
https://buildbot.suricata-ids.org/builders/inliniac/builds/63