All dates, ints, and amounts are parsed in account, block, comment, vote and witness

Account
* _parse_json_data improved and all missing ints and dates added
* json() adapted to _parse_json_data()
Block
* _parse_json_data added, expiration and timestamp are now datetime objects
* property json_transactions and json_operations added, which return trx and ops with date strings
* BlockHeader improved and datetime parsing added
Blockchain
* adapted to new datetime objects
CLI
* adapted to code changes
Comment
* int and dates inside active_votes are parsed
* Missing dates and int are parsed
Market
* date parsing to datetime added to market_history
Vote
* Some improvements
Witness
* _parse_json_data added
* all date and ints are parsed
* Demo added to Witnesses, WitnessesVotedByAccount, WitnessesRankedByVote and ListWitnesses 
beemgraphenebase/types
* datetime handling added to PointInTime
Unit tests
* test_account adapted
* test_export added to test_block
* test in test_comment fixed
* test_export added to test_witness