Fetch relative to provided URL and path

I access my Elasticsearch instance through a reverse-proxy. It pretty much looks like this: "http://some.host.com/es/(.*)" -> "http://localhost:9200/$1". The base of my elasticsearch instance is found at "/es/" rather than "/". 

If I make the following request:
```es2unix -u http://some.host.com/es/ health```
The current master will query "http://some.host.com/_cluster/health".

This patch adjusts the behavior so that the provided URL is treated as the full base path, and all URLs are fetched relative to that path. 
So, now, the command:
```es2unix -u http://some.host.com/es/ health```
Will query send a request to "http://some.host.com/es/_cluster/health", which makes me a very happy person.

Thanks!