Fix testing urls for SoundCloud
Fix urls in example project
VideoNode.get_backend with is_secure
Closes #11.

Major changes:

- filter ``embed`` is replaced by tag ``video``.
- removed caching for whole backend, caching properties instead.
- HTTP or HTTPS is set by context.is_secure() (just for ``video`` tag).
- backends can be HTTP only (``allow_https = False``).

Minor changes:

- changed theme of docs to RTD theme.
- help about testing HTTPS on development server.
- added and modified tests