Fixups

[mohawk2]

These are further minor fixups on the path to having "relations" work.

This closes #48.

[coveralls]

Coverage increased (+0.2%) to 99.541% when pulling 1fdf5ea on mohawk2:fixups into 9e7aece on preaction:master.

[mohawk2]

Notable changes in the last few commits:

• unify tests to all use %Yancy::Backend::Test::SCHEMA - rationale is that all the tests need to work with real backends, and this exposes any failure
• now if there is a single unique column that is not a PK (which I gather is called a "natural" key, like an SSN), x-id-field will be set to that over a "surrogate" (normal relational ID) integer one

[mohawk2]

I've made the backend tests apply exactly the same tests to the mock one, which needed a bit of tweaking. I also added @rmallah's idea (in #49) of handling list for boolean in the backends, and adjusted his commit to then use that, which you can see is now included!