add user and upstart job

QA: 
1. build binary deb - run "dpkg-buildpackage -rfakeroot"
2. install it - run "dpkg -i ../charmstore_0.0.1+201411*.deb"
3. charmd should be running as the charmstore user (assuming you happen to have mongod running on localhost:27017 too).  If you didn't have mongo then you should see the process attempt to start and die.

It will be the job of the charm to update the config so that it connects to the correct mongod.
This is a bit of a trick the charm uses to make sure it doesn't start without being properly configured, without needing complex logic in the hook. We need that to stay the same probably so this will only work if we don't automatically write a /etc/charmstore/config.yaml