Accessibility 418

Fixes #418

I have added alternative text to all images which did not have them.
I have fixed all the "Known Problems" and "Likely Problems" shown by http://achecker.ca on the index page - this will hopefully fix most of the problems throughout, as the same template is used to render the rest of the docs.

I looked into automated checking for various issues, but after discussion with @exarkun and @itamarst decided that the current situation of guidelines + fixes for the current most major issues was the best use of time.

The (best) automated testing options would not fit into our current BuildBot configuration well, as they would, I believe, take too much time to perform after each push - perhaps they could be . In particular I looked at https://pypi.python.org/pypi/robotframework-selenium2accessibility. Another problem with using an automated tool could be how we deal with externally written parts of our site. In particular, Disqus has numerous accessibility problems which we are not going to be able to fix. The likely best solution is to make the checking tools ignore these.

I researched into how words like "click" as mentioned in an above comment are viewed. The main objection is to links with text like "Click Below" which are confusing to those using a screen-reader for example.

I decided not to take on the task of enforcing valid HTML markup partly because it is a larger task than I hoped, and I don't think that it is worth it. @msh100 and I spoke about how the HTML could be more accessible, and he has said that for this and other reasons a rewrite should be considered, and that these principles could be taken into account then.