Ability to deploy units to specific machines.

Ability to deploy units to specific machines.

Support ToMachineSpec in goenv backend 
ServiceDeploy and AddServiceUnits calls.

Also update the pyenv for compatibility.

Drive by fix to code indentation/style.

QA:

Ensure the sandbox mode works well:
- make prod;
- deploy a service, add units, check
  everything is fine.

Live test the functionality on an ec2
environment:
- juju bootstrap -e ec2 --upload-tools
- juju deploy -e ec2 juju-gui --to 0
- juju expose -e ec2 juju-gui
- juju set -e ec2 juju-gui juju-gui-console-enabled=true juju-gui-source="https://github.com/frankban/juju-gui.git add-to-machine-spec"
- when the GUI is ready, log in, open the JS console;
- deploy a new service (e.g. mysql) as usual dragging the badge from
  the browser. This is done to ensure the usual way to deploy services
  from the GUI still works well: a new machine should be created to host
  mysql;
- add another unit to mysql using the inspector: at this point you should have
  mysql deploying to machines "1" and "2"
- using the console add a new machine, another new machine including an LXC
  container, and a new LXC on machine "1":
  app.env.addMachines([{}, {containerType: 'lxc'}, {containerType: 'lxc', parentId: '1'}]);
- use `juju status -e ec2` to ensure the new machines/containers have been 
  created: machine "3" and "4", containers "4/lxc/0" and "1/lxc/0"
- deploy wordpress to "1/lxc/0" by running the following:
  app.env.deploy('cs:precise/wordpress-21', 'wordpress', null, null, 1, null, '1/lxc/0');
- add another unit to wordpress, placing it on machine "3":
  app.env.add_unit('wordpress', 1, '3');
- define the following callback that will help visualizing
  the data returned by the API calls:
  var callback = function(data) {console.log(data)};
- ensure an error is returned when trying to deploy multiple units on
  the same machine:
  app.env.deploy('cs:precise/haproxy-27', 'ha', null, null, 3, null, '4/lxc/0', callback);
- deploy haproxy to machine "4" and container "4/lxc/0":
  app.env.deploy('cs:precise/haproxy-27', 'ha', null, null, 1, null, '4', callback);
  app.env.add_unit('ha', 1, '4/lxc/0');

Done, destroy your environment, thank you!