Create self-standing IPython Notebook Puppet module

- For the sake of portability, replaced dependency on 'systemuser' resource
  (from generic-definitions.pp) with equivalent user & group resources.

- Dito generic-definitions.pp's 'upstart_job' resource (which could stand to be
  improved..) Replaced with a proper service resource, specifying upstart as
  the provider, but also creating an init.d symlink to /lib/init/upstart-job.

- Added README, expanded parameter description, and documented default values.

- Change default IPYTHONDIR from '/var/lib/ipython' to '/srv/ipython', which
  better conforms to the Filesystem Hierarchy Standard.

- Changed ill-named 'graphs_dir' to 'mplconfigdir', in keeping with the name of
  the environment variable it shadows.

- Followed Puppet's style guide for specifying default values for parametrized
  classes that are based on other variables.

- In erb templates, use '@varname' rather than 'varname'. (The latter notation
  is liable to conflict with Ruby built-ins and is thus discouraged.)

I tested it using Vagrant and it works well.

Change-Id: I2db8206d91e929c943fddab8e09db60f458739b0