Allow "url" sub key in database.yml configuration

Currently a developer can pass in a YAML configuration that fully specifies connection information:

```
production:
  database: triage_production
  adapter: password
  pool: 5
```

They can also pass in a string that specifies a connection URL directly to an environment key:

```
production: postgresql://localhost/foo
```

This PR allows the use of both a connection url and specifying connection attributes via YAML through the use of the "url" sub key:

```
production:
  url: postgresql://localhost/foo
  pool: 3
```

This will allow developers to inherit Active Record options such as `pool` from `&defaults` and still use a secure connection url such as `<%= ENV['DATABASE_URL'] %>`. The URL is expanded into a hash and then merged back into the YAML hash. If there are any conflicts, the values from the connection URL are preferred. 

Talked this over with @josevalim

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