Fix blockquote style to make it work for both reST and markdown
Change CSS classes name for floating images to match those generated by reST figure and image directives

Markdown has no directive for captions so lets use what reST has
to offer.

Markdown users will have to add raw HTML anyway, to have captions with
their images. These classes can be used in it.
Floating image should not leave space between content boundary and its margin
Add CSS rules for reST image directive
Show modified date only if user has explicitly set it
Make sure that modified metadata is backward compatible

Pelican <=3.3 it is a string
Pelican >3.3 it is a datetime object

Close #69
Remove white spaces