Add tests using mocha+chai

Includes reduced test case for a bug mentioned here
https://github.com/OpenSocial/activitystreams.js/issues/3

I tried to dig in to fix it, but couldn't figure it out. :(

So the tests run with `npm test`, but they fail because of that issue. May be worth adopting some test infrastructure even with this test disabled for now. Then, for example, you could set up [travis](https://travis-ci.org/) for this repo for free.