Add option to [Read More] on main page, and truncation limit

[ryanstout]

[berkes]

Quick way, that I use, is a different partial for teaser-mode: _blog_post.html.erb used for full view, blog_teaser.html.erb for partial-view.
In fact, this would eliminate the _blog_post.html.erb alltogether, since that is used only in "show" and could therefore live in show alltogether.
It also remove the need for some if-statements (like wether or not to show comments) in the partial: making the partial a lot cleaner.