Add support for reconnect exponential backoff

[soerenbe]

Like discussed in #989 here the MR for the support for the exponential backoff reconnect delay.

As discussed this adds the additional parameter reconnect_backoff_max to WebSocketApp.run_forever

I was not 100% sure where to place the calculation of the reconnect timer, since it it should be called from the normal run_server loop and also from the custom dispatcher. I saw 2 possible implementations: A method for the WebSocketApp class or a function inside the run_forever method.

I chose to implement a simple method for the WebSocketApp class. There I could store the retry counter. Passing the retry parameters was ok for me. But overall this functions definitions inside the run_forever method felt a little strange.

Some random notes:

• If the value reconnect_backoff_max is smaller than the original reconnect timer, the value of reconnect_backoff_max will be used as reconnect time, but there will be also a warning message.
• I have added a documentation chapter about the reconnect behavior. This also includes a short description of the already existing reconnect parameter
• I added some basic tests for the calculation method. If you think there should be some end-to-end test, it would be nice If can give me some hints, because this tests with a running ws server looked a little confusing and hard to setup to me.

Looking forward to your feedback!
Sören