Proxy support for Dropwizard HTTP client

I often encounter with a situation when in closed corporate environments direct access to the Internet is blocked by security reasons. So all connections to external resources happen through a proxy server (usually with BASIC authentication).

Deploying applications in such environments becomes a huge pain, because usually you don't know in advance that an application will need proxy connection support and sometimes you just don't have possibility to change code to add this facility.

I believe, it would be quite useful to have this facility in the Dropwizard config. It frees an application developer from thoughts about environments where an application will be used and shifts this responsibility to operations guys. And they can configure proxy client support just in the application config without using external tools. 

PS I've added the docs to proxy configuration. But I'm not a skilled english speaker, so, please, fix them if I messed up something. 
I think it would make sense to call the other constructor to eliminate duplicate assignments which have to be maintained.

```
this(host, port, Optional.of(DEFAULT_SCHEME), Optional.absent());
```