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
Change CSS class from page_header to page-header

Fix #70
Change CSS class proj_desc to proj-desc

Fix #70
Change CSS class articles_timeline to articles-timeline

Fix #70
Change CSS class next_article and previous_article to next-article and previous-article

Fix #70
Change CSS class mc_embed_signup to mc-embed-signup

Fix #70