Added a cli command for user_delete

Did some light interactive testing, but need to figure out how to write
unit tests for the cli.
Add 'Getting Started' message to README.rst
Rephrase instructional code

- Now, instead of explicitly saying "you only need to do this once", I say for
  each command when you need to do it, using words like "each" and "the first
  time"

- More explanation of the Debian/Ubuntu install problem

- Remove needless warning for people using nonstandard shells.  (The
  instructions work correctly for bash, and with slight modification for zsh
  as well.  This covers all reasonable users.)
Only pip install once

As Ian/zenhack noted, the '-e' flag to 'pip' makes the install editable, so
only one install is required.
Cleanup haas.model

- ForeignKey references references are now to id, rather than label.
- Moved the id and label fields to a superclass
- Moved the __repr__ method to a superclass. We don't need the detail in
  the previous version, and it was hard to test (coverage tool was
  perfectly happy with the if expression that was in many of the
  individual methods we had before).
Change formatting of 'haas' in README.rst
Merge pull request #117 from gsilvis/doc-update-getting-started

Add 'Getting Started' message to README.rst
Add the missing 'group' to lots of stuff

This passes all existing tests.  (Note that 4 tests were removed, for the 2
API calls that were also removed.)
Merge pull request #121 from zenhack/model-cleanup

Cleanup haas.model
Merge branch 'devel' into api-group-ownership-consistency
Merge branch 'devel' into api-group-ownership-consistency
Remove bad comment, and make whitespace consistent
Make 'group' an argument to constructor when appropriate

i.e. for models where the group is mandatory
Make 'group' column non-nullable where appropriate
Added cli/server components of node_register
Added a test for duplicate node_register calls.

These should throw an error, and they do.
Added group_{add,remove}_user to haas.server and haas.cli.

Conspicuously, there's no group_create yet, so testing this manually is
a bit awkward, but can be done. It's on the TODO list.

Additionally, the functions had been omitted from the docs, so they've
been added there as well.
haas cli now prints out argument names as part of the help.

You now get output like:

    ...
    user_create <username> <password> # comment
    ...

instead of:

    ...
    user_create # comment
    ...

The arguments are pulled from the functions themselves.
The cli was using the wrong url for project_deploy, fixed.
Merge pull request #127 from gsilvis/api-group-ownership-consistency

Api group ownership consistency