Add Python 3 support.

This commit combines the output of the following:
    lib2to3.fixes.fix_except
    lib2to3.fixes.fix_idioms
    lib2to3.fixes.fix_import
    lib2to3.fixes.fix_ws_comma
    lib2to3.fixes.fix_xrange
    libmodernize.fixes.fix_dict
    Replace filter with list comprehension.
    Wrap map() in list/tuple, it's an iterator in Py3.
    libmodernize.fixes.fix_metaclass
    libmodernize.fixes.fix_next
    libmodernize.fixes.fix_raise
    Update imports using six.moves.
    raw_input
    Fix urllib
    Fix print handling (brackets, __future__).
    Fix unicode handling (remove u'', smart_str/text).
    Python 3 string related fixes.
    Update model string methods for Python 3.
Fix doctests.
Include typeof check in JSONP callback response.

This is more robust, and helps against attacks such as Rosetta Flash:
https://miki.it/blog/2014/7/8/abusing-jsonp-with-rosetta-flash/
Remove requirement for Python GDAL and memcached.

Add Python 3 testing to Travis, and work around issue with echo/YAML
indenting.
Use backport of Django 1.7's update_or_create.

Django 1.7.1 calls the built-in version by default on related managers,
so move to using a version that matches its behaviour more precisely.
Fix flake8 warnings.
Add new Christmas letters to Santa postcode.

This is the postcode now according to
http://www.royalmail.com/letters-to-santa