Reworking the documentation, Fix #471.

This work was done during the PyconFR sprint with @quantinvestor.
Update install documentation:

* be more friendly to people who are not familiar with Python
* add Python 3 versions
* update libzmq versions Ref. #566
* remove the list of dependencies which is listed in the
  pip-requirements.txt file and was out-of-date
Add syntax highlighting for all config examples
Move built-in plugins docs to the admin section

Using built-in plugins is not only for developers
Improve built-in plugins docs

* improve intro
* quote code snippets so that it shows a fixed-width font
* document "use" option for the WatchDog plugin
Improve docs on adding commands

* talk about waiting instead of async
* syntax highlighting
* typos
Improve documentation on writing plugins

* link to the built-in plugins page instead of giving an incomplete list
  here
* syntax highlighting
Improve hooks documentation

* change the title : we don't *write* hooks, we use them to write hook
  handlers
* move the list of hooks at the top, as it will often be used as a
  reference
* merge paragraphs that were just one sentence long
Simplify classes section title

"classes collection" is incorrect (eg. we'd say "a book collection", not
"a books collection") "class collection" would be correct but I find it
slightly confusing. Why not just call this section "Classes".
Improve homepage

* syntax highlighting
* mention the web interface in the intro
Mention Python 3 support in Step By Step section
Merge pull request #622 from amarandon/pyconfr-doc-rework

Pyconfr doc rework