Use doctest for examples, improve docs

Account
* Example forget SimilarAccountNames  added
* Example for history and history_reverse improved
Amount
* Example is tested with doctest
Block
* Example is tested with doctest
Price
* Fix example
Profile
* Fix example
Steem
* Example is tested by doctest
Transactionbuilder
* Example is tested by doctest
Vote
* datetime.now() changed to datetime.utcnow()
* Example is tested by doctest
Witness
* datetime.now() changed to datetime.utcnow()
* Example is tested by doctest
Docs
* Installation instruction improved
* Quickstart created
* Discord channel added to support