history and history_reverse improved by estimate_account_op()

Account
* print_info improved (Last Vote added)
* entryId changed to start_entry_id in get_feed, get_blog_entries and get_blog
*rpc.get_account_history moved to _get_account_history
* estimate_account_op added, can be used to fastly get account op numbers from dates or block nums
* history and history_reverse uses estimate_account_op
Exception
* KeyNotFound removed
Memo
* raises MissingKeyError instead of KeyNotFound
Price
* small improvement
Steem
* __repr__ added
Transactionbuilder
* get_potential_signatures, get_transaction_hex and get_required_signatures added
* raises MissingKeyError instead of KeyNotFound
VotesObject
* __contains__, __str__ and __repr__ added
Wallet
* raises MissingKeyError instead of KeyNotFound
Witness
* __contains__, __str__ and __repr__ added to WitnessObject
Steemnoderpc
* small improvements
Operationsid
* producer_reward added
Unit tests
* new function added and tests adapted to changes