Added blog_key function that is useful to achieve strong db consistency

See: https://developers.google.com/appengine/docs/python/datastore/structuring_for_strong_consistency
Added last_modified field to Post entity
Replace newlines with html <br> tags in post content
Move css styles to a separate file
Fix typo and cleanup
Make render_str global
Added more jinja templates
Renamed NewPostHandler to NewPost
Add new BlogFront instead of MainPage
Better post handling and cleanup front and newpost templates