Add proxy support to SSH connections

The SSH wagon lacks support for connections made through HTTP/SOCKS proxies. I think this support shouldn't be difficult to add as the SSH wagon internally uses JSch, that provides support for these types of connections, as you may see in the following examples:

http://www.jcraft.com/jsch/examples/ViaHTTP.java
http://www.jcraft.com/jsch/examples/ViaSOCKS5.java

AbstractWagon already has a proxyInfo field, so I think the proxy information could be extracted from here, and used as shown in the examples.