Full support for different DCs and ++stable

Scheme layer used: 66

Working with different data centers finally works! On a different order of things, reconnection is now performed automatically every time Telegram decides to kick us off their servers, so now Telethon can really run forever and ever! In theory.

Another important highlights:

• Documentation improvements, such as showing the return type.
• The msg_id too low/high error should happen less often, if any.
• Sleeping on the main thread is not done anymore. You will have to except FloodWaitError's.
• You can now specify your own application version, device model, system version and language code.
• Code is now more pythonic (such as making some members private), and other internal improvements (which affect the updates thread), such as using logger instead a bare print() too.

This brings Telethon a whole step closer to v1.0, though more things should preferably be changed.