Customizable type registry.

[isoos]

The is the first public-facing change for #346, enabling the customizable type converters to be added to a connection through the TypeRegistry.

[isoos]

/cc @simolus3 for generic comments
/cc @insinfo I think this would enable your use to proceed without much further changes on the package:postgres, and we could make it the first type extension package that is not tightly bound to the base package. I would expect similar extension for postgis types.
/cc @wolframm I think the next step would be the cache for OID values, otherwise we are close to proceed with your PR, but I'm afraid it still needs a few breaking changes in the public API to get it done.