Add reconnection & backoff logic to client mode

[smartin015]

• Adds randomized exponential backoff reconnection to server when running in client mode and the connection is closed
• Splits business logic of index.js into initial rclnodejs setup and websocket+bridge connection handling
• Fixes typo in README
• Moves "shutdown" behavior into a single function
• Simplifies bridge error reporting (no longer requiring bridge ID to be passed, since we know by context which bridge we're listening to).

[smartin015]

As a side note - thanks @minggangw for the timely review of all of these PRs. It really increases my motivation to upstream them :)

[minggangw]

You may know we have developed the rclnodejs client for ROS2 on which this bridge bases, and because of the limitations of rosbridge v2 Protocol (you know it's not for ROS2 originally), it's difficult to add some functions which have already been implemented in rclnodejs. So, I suggest you could use Electron or nw.js with the support of Node.js & Web to experience full features of ROS2. Also, you are welcome to contribute directly to it, thanks!

[smartin015]

Thanks for the tip! All I need for now is topic and service calls accessible via websocket, but if I need anything more complex than that I'll definitely look into that.

[minggangw]

@smartin015 0.3.0 has been published, thanks for your contributions!