Add cmd/server package to enable hosted Command execution via RPC

prereq: lp:~fwereade/juju/go-tweak-supercommand

This is all new code, and shouldn't impact anything else, so it should be
adequately explained by the inline comments (and if it isn't, those should
be fixed). Nonetheless, the general idea is as follows:

1) Hook commands need to run the "meat" of their code on the server.

2) The python implementation includes an awful lot of plumbing to support a
   mechanism whereby each individual command marshals relevant params to a
   server and gets back results which are then formatted for stdout; in
   juju.hooks, the cli, commands, and protocol modules all contain a lot of
   code purely to support this approach.

3) By implementing a single server method that takes a command line, a
   working directory, and a context ID; and returns an exit code and the
   command's output: at a stroke, we eliminate the vast majority of the
   marshalling busywork, and become able to implement each hook tool as a
   single Command, which can have direct access to the hook context in which
   it will execute.

https://code.launchpad.net/~fwereade/juju/go-cmd-server-package/+merge/96140

(do not edit description out of merge proposal)

[fwereade, 2012-03-06 14:57:30]

Please take a look.

[fwereade, 2012-03-06 16:20:17]

Please take a look.

[fwereade, 2012-03-06 16:25:59]

Please take a look.