Display deprecation warning for ``FauxFactory``.

Now when someone imports ``FauxFactory`` and uses any of its 'older'
functions, a deprecation warning message will be displayed as shown
below:

```python
In [1]: from fauxfactory import FauxFactory
fauxfactory/__init__.py:723: Warning: The FauxFactory class is deprecated. Please use functions from the fauxfactory module instead.
  category=Warning)

In [2]: FauxFactory.generate_alpha()
fauxfactory/__init__.py:696: Warning: Function generate_alpha is now deprecated! Please update your your code to use the gen_alpha function instead.
  warnings.warn(warn_message.format(name, new_name), category=Warning)
Out[2]: u'miRSshikjd'
```

Tests included. Closes #51.