Added overriding 'uri' property to config object

I added optional config.uri property that, if defined, overrides other config properties. This allows much easier deployment as you can define a single env variable with URI instead of three (host, port, db name) or more.

It also allows the use of the library with Cloudant which doesn't use database names but URIs of https://<user name>:<password>@<Cloudant account>.cloudant.com and distinguishes different databases based on the user name.