Explain service adaptor testing, include bX service adaptor test, fix fixtures

I updated the fixtures to not use ID since it was unnecessary.

All that noise about Travis parsing fixtures differently in the commit comments below is garbage.  [It may be true](http://about.travis-ci.org/blog/2013-01-31-post-mortem-build-outage/), but it wasn't my issue.  I needed to include the [`:referent_values` fixture](https://github.com/team-umlaut/umlaut/commit/a927b5ecb37ccb4911472123fdb7334c64b962dc#diff-cc85d28c801f66371b3be5cef19f69d8L9) in my test.  That was all.  

I've also included a unit testing README explaining how to test service adaptors.

And, of course, I've included a bX test (with VCR cassette).