Fixed the 'cannot find file bootstrap' bug

Dear klenis:

Main changes (1 & 2 fixing bug 'cannot find file bootstrap' ):
1. In Gemfile, add
    gem 'autoprefixer-rails'
2. Rename application.css to application.css.sass and add
    @import "bootstrap-sprockets"
    @import "bootstrap"
3. Enable both title and body editing.

I've integrated your contribution to my fork of bootstrap-todo-app by  Brandon Conway at https://github.com/GuanglinDu/bootstrap-todo-app. 

One known bug: after creating a new post, the index page has to be refreshed to enable the editing.

Thank you so much
Guanglin Du