'List (seen) task types
**Example request**:
.. sourcecode:: http
GET /api/task/types HTTP/1.1
Host: localhost:5555
**Example response**:
.. sourcecode:: http
HTTP/1.1 200 OK
Content-Length: 44
Content-Type: application/json; charset=UTF-8
"task-types": [
"tasks.add",
"tasks.sleep"
:reqheader Authorization: optional OAuth token to authenticate
:statuscode 200: no error
:statuscode 401: unauthorized request'

@web.authenticated
def get(self):

    seen_task_types = self.application.events.state.task_types()
    response = {}
    response['task-types'] = seen_task_types
    self.write(response)
