Basic Support for Python 3

[BeryJu]

Imports & everything else adjusted to python3, works with both python 2 and 3. Unit tests succeed for 2.7, 3.4, 3.5 and 3.6: https://travis-ci.org/BeryJu-org/netbox

[jeremystretch]

This is great, thanks!

Django will bump up to 2.0 after 1.11, at which point support for Python 2 will cease. I'd like to continue supporting Python 2 until then to give users some time to transition. The documentation will need to be adjusted to reflect that both Python versions are supported.

Also, for settings.py, let's just use from netbox import configuration instead of repeating netbox.configuration so many times.

I'm still debating what to do about ldap_config.py: It would be nice to absorb it into configuration.py but there's an argument to be had for keeping it separate as well. At any rate, let's separate that change from this PR as it's not relevant to Python 3.

[BeryJu]

I also have to test this more on python 2 as I think using __str__ brakes the interface because python 2 wants __unicode__.

[jeremystretch]

I think we need the python_2_unicode_compatible() decorator: https://docs.djangoproject.com/en/1.10/topics/python3/#str-and-unicode-methods

[BeryJu]

I was looking for something like python_2_unicode_compatible but didn't find anything. I have added to every model that defines a __str__ in a612012.

[jeremystretch]

Looks good to me. FYI this is commit number 1000!