Add ability to specify path and/or additional headers for "emit" request.

[alexdebrie]

Subscriptions are set up on a path, so events may need to be emitted on a path.

Further, a user may want to include additional headers with the emit() request, such as an Authorization header used for Auth.

A proposed update to the emit() signature could be:

eventGateway.emit(<CloudEvents object>, <Options>)

The usage would look like:

eventGateway.emit({
  eventID: '1',
  eventType: 'user.created',
  cloudEventsVersion: '0.1',
  source: '/services/users',
  contentType: 'application/json',
  data: {
    userID: 'foo'
  }}, {
  path: /users,
  headers: {
    "Authorization": "Bearer 1234"
  }
})

[mthenw]

Yeah, makes sense.