Right now adapters have to expose a rather byzantine API:

  - They must be defined in `active_record/connection_adapters/<name>_adapter`
  - They must define `ConnectionHandling.<name>_adapter_class`
  - They must define `ConnectionHandling.<name>_connection`

All this is not very DRY and a bit annoying. Additionally it makes
it very hard to define aliases (e.g. `mysql` => `trilogy`), or to
substitute a default adapter for a specialized one.

This refactor aims at making all this easier by exposing a simple
`register` method, that third party adapters can call from a Railtie.