Make the user lookup URL configurable

This commit addresses the hardcoded URL to lookup a user on a 3rd party site. Simply
define a lookup_url in the contact section of the config, using a %s where the username
should be substituted, eg. http://foo.com/bar/%s
adding tests for new contact lib functions
Merge pull request #20 from nkammah/configurable_contact_url

Make the user lookup URL configurable