non-freshing join
1,redirect to login page while click "join" button.
2, refresh count of joined.

know issue:
  1, while no guys joined an event, this first guy click "join" button, the "Joined guys" section should not shown.
  2, avatar section could not refresh after someone joined.
1, cancel a joined event.
2, read tooltip text from i18n.
优化代码
add js testing for asyn event joining.
use angular services to reload participated users.
support capybara javascript feature test.
fixed rspec error. add angular services test.
Merge pull request #329 from saberma/saberma-feature/319-join-with-no-fresh

报名后更新「已参加用户」 #319