Add ability to configure URL Prefix

This commit allows a user to configure per-node URL prefixes.  Very
simple change, but touched many places of the code (particularly the
older unit tests).

Connections are now built with a single array containing [host, port,
path].  Port and path are optional.

The user enables prefixing by adding it to the host string parameter
on client creation:

```
$params['hosts'] = [
  'localhost:9200/prefix',
  'localhost:9201',
  'localhost:9202/another/prefix'
];
$client = new Client($params);
```