Added support for local source address to TcpOutput plugin

This patch adds support for a local address config option for the tcpoutput plugin that will bind to a specific local IP address.

Unfortunately this won't work with TLS connections for now. 

Once golang 1.3 releases this patch will be rolled in:
https://code.google.com/p/go/source/detail?r=3d37606fb79393f22a69573afe31f0b0cd4866e3&name=default

And tls.DialWithDialer can be used to add support easily for tls. For now this option is ignored on TLS connections.