Support for canonical urls

Adds a <link rel="canonical" href="..."> to each page for a better google search result (in the future). Outdated documentation versions should no longer come as first entry in search results.

e.g. <link rel="canonical" href="http://docs.pylonsproject.org/projects/pyramid_tutorials/en/latest/getting_started/pyramid_setup.html"/>

This can be used for all docs based on pylons themes. 

And actually the update won't change anything unless `conf.py` of each doc is updated.

For example:

    html_theme_options = dict(
      github_url='https://github.com/Pylons/pyramid_tutorials',
      canonical_url='http://docs.pylonsproject.org/projects/pyramid_tutorials/en/latest/'
    )