Full rewrite

Instead of returning one URI connection string, `mongoify()` now returns a dictionary of all URI connection strings found in the environment settings, falling back on the default argument's value if one is provided. From the docstring:

    This method will look for MongoDB URI connection strings in the
    environment settings. It will first check for ``MONGO_URI``. If this
    setting is found, it will be in the result ``dict`` as the
    ``default`` key. The method will then iterate through the
    settings looking for any additional URI connection strings. Any that
    are found will be added to the result ``dict``.

    In the event that ``MONGO_URI`` was not found, the first URI
    connection string encountered will be used as the ``default`` key.
    This URI will appear twice.

    In the event that ``MONGO_URI`` was not found and no other URI
    connection strings were not found, the value provided through the
    ``default`` argument will be used as the ``default`` key.

This change results in a more useful tool with much leaner tests.