Set a default page title.
Add OG stuff for section, speaker and speech_detail pages

Also::
* Move the title generation to the view so that we can easily use it for
both the og_title and the page_title.
* Change to using bleach rather than vanilla striptags.
Override MEDIA_ROOT as well as MEDIA_URL.

This has two desirable effects:

1) The files created by the tests are in a temp directory which is
deleted at the end, so don't end up cluttering up the real MEDIA_ROOT
directory.

This could be done better with a context manager - probably in a class
decorator, but the temporary directory should be deleted unless things
go wrong.

2) FileSystemStorage keeps its own copy of MEDIA_URL, so
override_settings(MEDIA_URL=...) doesn't make all the tests pass all
the time - exactly which ones fail and when seems to vary depending on
the system.
Skip tests which need override_settings(MEDIA_ROOT) in Django 1.4.

override_settings(MEDIA_ROOT) isn't made to sort out FileSystemStorage
until Django 1.5: (https://github.com/django/django/commit/a24ffa52d029)
so let's skip tests that need that in 1.4.

In speaker_tests, instead of skipping the whole test, which does other
useful stuff, I've just put an if around an assertion.
Get correct urls in OpenGraph section.

These URLs for OpenGraph need to be absolute
Using request.META.HTTP_HOST saves us from worrying about getting
the instance prefix when needed.
Allow testing of OG keys against regular expressions.

Instead of just a string to check for equality, you can now pass
a compiled regular expression as the test for an opengraph key, which
will then be searched for in the content for that key.

Additionally, check that any OG key which should contain a URL
has an absolute url if it in there.
Check OG stuff for speaker_detail and make image url absolute.
Fix up OG stuff on speech detail page and test.

In particular, get rid of quoted speechmarks around the title and
get the image URL absolute.
Add some testing and description for OG in sections.
Fix string decode error.