Update to work with pelican 3.4.

The latest version of pelican has made some changes that break this plugin
(read_file function removed from readers module). This commit fixes things up
to use a Comment class that properly derives from the Content base class, and
adds some additional settings documentation to get rid of warnings.
Restored metadata validation
Removed tab indent
Merge pull request #3 from russgray/master

Update to work with pelican 3.4