Make it easy to run linters and unit tests

Add a make file containing the following targets:

* help
* lint
* test

List out development dependencies in `requirements-dev.txt`. Mention both the
make file and requirements file in the readme.

Both the linters and unit tests pass:

    $ make lint
    No config file found, using default configuration
    $ make test
    python -m unittest discover --start-directory tests --top-level-directory .
    ..............No handlers could be found for logger "nailgun.client"
    ................
    ----------------------------------------------------------------------
    Ran 30 tests in 0.006s

    OK