Connections - Adds tests and documentation

[NeedleInAJayStack]

This adds tests and documentation to the Connection system, which should improve stability, maintainability, and adoption of this feature.

[cshadek]

Should there be a test for when a connection is a field on a Type, or is that not needed?

[NeedleInAJayStack]

@cshadek Yeah, I originally had them as object fields, but it made these tests pretty verbose. I've added one here
#109 where I fix the Connection aliasing issue you reported.

[d-exclaimation]

LGTM!