Python 3 compatibility

[drcursor]

I installed this using pip3 and ran around in circles trying to solve the following error :

>>> from pykeepass import PyKeePass
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ImportError: No module named 'pykeepass'`

Took me some time to figure out that this project is only running correctly under Python 2.

This should be made clear on the documentation.

[pschmitt]

I added a line to the README in 96ad692. I'll leave this issue open nonetheless since Python 3 compatibility is a must have in my opinion.