Ensures all ssh commands are run through a login shell

Makes gusteau consistent with Vagrant, which by default wraps all SSH invocations (eg chef-solo) in "sh -l" 
Fixes #40

Disclaimers:
* Only tested under vagrant; 
** never tested without sudo; never tested with RVM
* might want to make it configurable, like vagrant does
* might want to use `bash -l` instead of `sh -l`, to be consistent with vagrant, and to pick up rvm .bash_profile

Background reading:
* http://stackoverflow.com/questions/216202/why-does-an-ssh-remote-command-get-fewer-environment-variables-then-when-run-man
* https://github.com/mitchellh/vagrant/blob/master/config/default.rb
* https://github.com/mitchellh/vagrant/blob/master/website/docs/source/v2/vagrantfile/ssh_settings.html.md