Part two of client work.

Adds functions for submitting commands to a node, and getting a list of nodes from the system.

Open questions:

1. Right now the types are all of type 'string', should we alias these to something like 'Command' ?

2. Should the client maintain a list of valid nodes which may be sent commands based on the last run of the list node function?
(Tip) Go's style is to start function and method comments with the name of the function or method like: "NewClient creates a new..."

> Doc comments work best as complete sentences, which allow a wide variety of automated presentations. The first sentence should be a one-sentence summary that starts with the name being declared.

http://golang.org/doc/effective_go.html#commentary