implement connected property

in favor of the (now private) assureConnected method. the connected property return a promise that resolves when the connection has been established so you can do something like connection.connected.when((connection) => { connection.sendRequest(...) }). this ensures that we are not sending requests before we are connected to the backend module

any other API refactoring with Promises as described in #3 (closed) does not make sense as we need to store the callbacks and resolve them when the incoming message arrives.

closes #3 (closed)