Feat/python3 update

[djstein]

Huge Update. Getting back to PEP 8 formatting, remove relative imports, support for Python 3. Leaving my name as author on this branch due to possibility it will never get merged and it being used in our production.

[stewartjarod]

This would be great 👍

[jdennes]

I don't use this library, and don't really maintain it either. It is supposed to be officially supported by Campaign Monitor.

These changes look good to me though, and I'm happy to merge this since I still can. If you want a faster response to contributions like this in future, I would recommend contacting Campaign Monitor support about the maintenance plan for libraries like this.

@djstein Thanks for making this better, and you can expect a new release shortly.

[jdennes]

Release: https://github.com/campaignmonitor/createsend-python/releases/tag/v4.2.2

[xfxf]

There appears to be a problem with this release. Can't install 4.2.2 on 3.6.2 on OS X, or 3.4.2 on Linux. pypi shows 'Unknown' as latest version - https://pypi.python.org/pypi/createsend/4.2.2

Same issue:

Collecting createsend==4.2.2
  Using cached createsend-4.2.2.tar.gz
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-build-fx46u_6d/createsend/setup.py", line 3, in <module>
        from createsend.createsend import __version__
      File "/tmp/pip-build-fx46u_6d/createsend/createsend/__init__.py", line 20, in <module>
        from createsend.version import __version__
    ImportError: No module named 'createsend.version'
    
    ----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-build-fx46u_6d/createsend/

Installing 4.2.1 works fine.