Added feature that allows for truncating the post body text.

This feature allows for truncation of the post body text at a
user-defined point for display on the home page. See the help text
when creating or updating a post in Enki's admin area.

Thanks to user mcary for his contribution.

Conflicts:

	app/views/admin/posts/_form.html.erb
	app/views/posts/index.html.erb