Support arbitrary db-spec k/vs from query params in URIs.

This makes it a lot easier to support SSL connections from URI-style
DB connection parameters.
Add explict tests for URI-style DB specs.

Note that there is some redundancy here since URI specs may be tested
by `postgres-str-db` and friends, but it wasn't clear how those are
invoked in the test suite as it is, so I've left them alone even
though they appear unnecessary.
Merge pull request #35 from technomancy/query-params-on-uri

Support arbitrary db-spec k/vs from query params in URIs.