can set category of lesson post
configure new relic
upgrade to rails 4.1.1
use Ghost-Markdown-Editor to implement checkin's markdown editor
fix some problems;

- remove all the inline styles and move them to cohorts/checkin.css
- delete `include_javascript` and `include_stylesheet` helper
  methods,include assets directly in application.html.slim
- comment `updateWordCount` function call in `ghostdown.js`,because it is
  useless for counting chinese words.
- remove `checkins_controller.rb`
- fine tune some styles.
Merge fate-lovely-markdown-editor

Conflicts:
	Gemfile
	Gemfile.lock