Introduce Reactive API

[ali-ince]

This PR brings reactive streams support into javascript driver.

1. Adds istanbul and nyc packages to enable code coverage statistics in nodejs (use NODE_ENV=test nyc jasmine to run tests with code coverage enabled).
2. Refactors stream observers used in sessions, connections and transactions and makes them part of protocol implementations which will enable us to override certain behaviours based on protocol versions.
3. Introduces RxSession, RxTransaction and RxResult classes that will be used for reactive streaming.
4. Updates README to cover more usage scenarios.

Resolves #271.