Shortcodes for YouTube and iframes

Added shortcodes 2.5.0 as dependency; Earlier versions corrupted
non-ASCII characters, see
https://github.com/dmulholland/shortcodes/issues/6

The rendered elements have a `shortcode` CSS class.

The YouTube shortcode supports various ways to refer to a video:

    - `{youtube VideoID}`
    - `{youtube youtube.com or youtu.be URL}`

URLs containing an '=' should be quoted, or otherwise the shortcodes
library will parse it as "key=value" pair.

The IFrame shortcode supports the `cap` and `nocap` attributes. `cap`
indicates the required capability the user should have in order to
render the tag. If `nocap` is given, its contents are shown as a message
to users who do not have this tag; without it, the iframe is silently
hidden.

`{iframe src='https://source' cap='subscriber' nocap='Subscribe to view'}`

Merged test code + added HTML class for shortcode iframes

setup.py

@@ -54,6 +54,7 @@ setuptools.setup(
         'Pillow>=2.8.1',
         'requests>=2.9.1',
         'rsa>=3.3',
+        'shortcodes>=2.5',  # 2.4.0 and earlier corrupted unicode
         'zencoder>=0.6.5',
         'bcrypt>=2.0.0',
         'blinker>=1.4',