Fbchat

Add MQTT over WebSockets support, fixing a lot of long standing issues. See 494 for more.

This is a fairly big change, so a few issues is to be expected, please report them if you find any!

- Fix an issue with determining the mimetype of external files, 471

- Fix 469, a python 2 specific issue

- Internal improvements, see 464

- Add `Client.fetchThreadImages`, see 434
- Clean up API documentation, see 454
- Fix documentation spelling, see 455

- Fix a few login issues, see 442 and 446
- Add heart reaction, see 445, thanks to kugo12