Fix #215: on before/after events, use same evtData

In case there's a need for other hooks, please
open a new issue or reopen #215.
Fix copyright header
Fix #214: improve events to be canceable

- Add new canceable event `candy:view.room.before-add`
- Add new canceable event `candy:view.private-room.before-open`
- Make `candy:view.message.before-send` canceable
- Make `candy:view.message.before-show` canceable
Add sourcemap to candy.min

Might help with debugging.
Merge dev