Add url_name to instance

We want to allow linking to instances with urls like
http://opentreemap.io/philly rather than http://opentreemap.io/12. I
added a unique url_name property to the Instance model that uses a
RegexValidator to ensure a url-friendly value.

The RegexValidator requires all lowercase letters to avoid
confusion, since URLs are case insensitive.
Modify instance urls to contain name not id

We want to have instance urls like http://opentreemap.io/philly rather
than http://opentreemap.io/12.
Merge branch 'master' into topic/instance-url-name

Conflicts:
	opentreemap/opentreemap/urls.py