make activities can create & edit & show & list
make user can register activity
Merge branch 'feature/activity' of https://github.com/zlx/union into feature/activity

Conflicts:
	app/models/user.rb
	config/locales/views.zh-CN.yml
fix create activity bug
Merge branch 'feature/activity' of https://github.com/zlx/union into feature/activity
markdown support for activity description
add datetime picker plugin for activity start time
Merge pull request #1 from williamherry/feature/activity

活动详细markdown和时间选择插件
add validate for activities and change Gemfile source to ruby
Merge pull request #190 from zlx/feature/activity

william组  活动发布－活动报名功能