Update shout.js

Change the prefix for shout as it might cause conflicts with other modules.
Now shout are able to be nested in a message. cf #32

###Example of use with userlist module activated:
- @pigeon
=> http://i.imgur.com/iOBmtNk.gif

- Sorry but I have @notime Memoria
=> Memoria: http://goo.gl/exuKV

- Hey did you know a lot of @pigeon can cause you a @mindblown ? ( @hello Memoria )
=> http://goo.gl/exuKV
=> http://goo.gl/vuEy9
=> Memoria: http://i.imgur.com/6YbAxG8.gif

###Example of use WITHOUT userlist module loaded:
- @pigeon
=> http://i.imgur.com/iOBmtNk.gif

- Sorry but I have @notime Memoria
=> Memoria: http://goo.gl/exuKV

- Hey did you know a lot of @pigeon can cause you a @mindblown ? ( @hello Memoria )
=> can: http://goo.gl/exuKV
=> ?: http://goo.gl/vuEy9
=> Memoria: http://i.imgur.com/6YbAxG8.gif