Switched to node-ssi

I switched to node-ssi and then found couple of issues in node-ssi. So I fixed that and set connect-ssi to point to my fork until my changes get merged into node-ssi. Presumably they'll get merged in soon and we can go back to the npm version of node-ssi.

Since this is a fairly major change, I also added some unit tests. And I added .travis.yml file so that you could log in at http://travis-ci.org/ and turn on automatic testing for pushes and pull requests ;)