Support Smarter Unix URL Scheme

Fixes #1166 

- No longer validate UNIX URLs by attempting to connect to every possible combination of hosts and paths
- Now possible to bring `_buildRequest()` logic back into the `init()` method

So I went the "smarter URL scheme" route, and more or less copied the work done in ngx_http_proxy_module. We now use the URL scheme `'http://unix:SOCKET:PATH'`. No new option required, and the change is much less severe.
```
OLD: request.get('unix://absolute/path/to/unix.socket/request/path')
NEW: request.get('http://unix:/absolute/path/to/unix.socket:/request/path')
```

An error is thrown if you use the old format.

/cc @mikeal & everyone from #1166