Make constants constant, fix broken ref in docs

Use tuples instead of lists in module `fauxfactory.constants`. The built-in
tests function correctly:

    $ make test
    python -m unittest discover --start-directory tests --top-level-directory .
    .................................................................................................................................................................
    ----------------------------------------------------------------------
    Ran 161 tests in 0.934s

    OK

Make some changes to documentation:

* Drop some comments from `fauxfactory.constants`. An explanation of lorem ipsum
  seems unnecessary, for example.
* Move remaining descriptions in to the module-wide docstring. This allows
  descriptions to be found both by Sphinx and the `help` built-in. Also include
  the `fauxfactory.constants` module in the API documentation. These changes
  allow documentation to be built with zero warnings.
Merge pull request #59 from Ichimonji10/constants

Make constants constant, fix broken ref in docs
Make gen_netmask verify function arguments

Make `gen_netmask` raise a `ValueError` if `min_cidr` is less than 0 or
`max_cidr` is greater than 32. Add tests for this boundary checking code.

    $ make test
    python -m unittest discover --start-directory tests --top-level-directory .
    .................................................................................................................................................................
    ----------------------------------------------------------------------
    Ran 161 tests in 1.508s

    OK

Fix #57.
Merge pull request #60 from Ichimonji10/gen-netmask-args

Make gen_netmask verify function arguments