Add Preview functionality

 - Add preview method that parses body of tip and turns plain text into Markdown
 - Add Post request to tips.js.coffee
 - Add preview route to routes
 - Re-format tip form
 - Add preview.js to handle Post response
Add character countdown functionality

 - Add jQuery to count how many characters the user has left to input
   into the title and description forms.
 - Add jQuery to change color as character count lowers.
Merge branch 'counter'

Conflicts:
	app/assets/javascripts/tips.js.coffee
Add jquery.turbolinks to Gemfile

 - Add jquery.turbolinks to Gemfile to solve issues with reload of
   javascript on new tip page.