Add support for loading Twitter contacts for invitation messages

This commit adds support for the PZH Web Server to query
Twitter to find out the user's twitter contacts and then
potentially send them a message.  The message sending has not
been implemented yet.

Jira issue: WP-1064
Initial set of changes to enable token-based cert exchange.

This contains a number of tests that need to be removed.

Jira issue: WP-852
Removed comments and added direct message support

If you are logged in with Twitter, you can now send
direct messages with URLs for invites.

Jira issue: WP-1064
Introduced token deletion for invitations

Jira issue: WP-1064
Merge pull request #9 from jplyle/cert-exhange-via-twitter

Support certificate exchange via invitation tokens, distributed over Twitter