Fixes GH saltstack-formulas/nginx-formula#14.

* Fixing iteration for building nginx by source with respect
  to modules
* Adding `CHANGELOG.rst` and `VERSION` per https://groups.google.com/forum/#!topic/salt-users/zHguqsk-mds

* Ensure `common.sls` references correct template filename
* Use `pillar.get(...)` for message_do_not_modify with default

* Re-organize according to http://docs.saltstack.com/topics/conventions/formulas.html#writing-formulas