Python 3 support in build_html_iframe()

[trawick]

build_html_iframe() uses dict.iteritems() and unicode(), which aren't part of Python 3. This patch uses six to make the code work under Python 3.

Testcase:

from aldryn_video.models import OEmbedVideoPlugin
o = OEmbedVideoPlugin()
o.oembed_data = dict(provider_name='foo')
o.iframe_width = "800"
o.iframe_height = "600"
o.oembed_data['html'] = '<iframe src="http://www.w3schools.com" frameborder="1" name="foo"></iframe>'
o.html

Expected result:

'<iframe frameborder="1" height="600" name="foo" src="//www.w3schools.com" width="800"></iframe>'

(or equivalent Unicode literal with Python 2)

[trawick]

AFAICT the failure of some of the Travis builds has nothing to do with this pull request. Here's a common one:

  File "/home/travis/build/aldryn/aldryn-video/.tox/py34-dj17-cms32/lib/python3.4/site-packages/cms/utils/placeholder.py", line 9, in <module>
    from django.template import TemplateSyntaxError, NodeList, Variable, engines
ImportError: cannot import name 'engines'
ERROR: InvocationError: '/home/travis/build/aldryn/aldryn-video/.tox/py34-dj17-cms32/bin/coverage run test_settings.py'

[FinalAngel]

@trawick we fixed some issues on the test environments. Can you rebase to master and push again?
Additionally please create an entry in the Changelog (1.0.1) for your changes.

I'll see to it that it gets merged once adapted :)