Allow pendulum comparison to None.

[jkeyes]

A comparison between a Pendulum object and None raises the following error:

>>> import pendulum
>>> pendulum.now() == None
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/vagrant/.pyenv/versions/server/lib/python2.7/site-packages/pendulum/pendulum.py", line 1129, in __eq__
    return self._datetime == self._get_datetime(other)
  File "/home/vagrant/.pyenv/versions/server/lib/python2.7/site-packages/pendulum/pendulum.py", line 2159, in _get_datetime
    raise ValueError('Invalid datetime "{}"'.format(value))
ValueError: Invalid datetime "None"

To fix, I modified _get_datetime to return None if value is None.