add/remove signals code/tests.
Add support for returing `false` from setup/add/remove/teardown handlers.
Fixed wrong emitter runner used for setup/teardown/on/off.
Signal add/remove should receive the same arguments as with on/off.
Added tests for prevent registering listener via add/remove.
Send same args to setup/add/remove/teardown (almost)
Add JSDoc `@preventable` to setup/add/remove/teardown signals.
Update JSDocs for setup/add/remove/teardown
Merge pull request #151 from troopjs/feature/event-signals

Added add/remove event signals.