Add notes to docs about filters

Based on my own experience documented in #120 and #121
Merge pull request #123 from untitaker/website

Add notes to docs about filters
Adding precisions about the "None" right management setting.

Also, "everybody has" is the correct English sentence.
Adding an hint for PAM authentification on Linux.

As said, if PAM is configured to check against the system files, the user running Radicale must have the proper permissions.
Merge pull request #139 from mike-perdide/patch-2

Adding an hint for PAM authentification on Linux.
Merge pull request #138 from mike-perdide/patch-1

Adding precisions about the "None" right management setting.
Add support of Python 3.4 in documentation
Merge branch 'website' of github.com:Kozea/Radicale into website