Improved beempy keygen and new update_account and beempy changekeys

* Add hive node
* update_account function added to blockchaininstance
* normalize added to PasswordKey, so that a Brainkey can be set as PasswordKey
* Fixed vote percentage calculation when post rshares is negative
* new beempy command changekeys
* beempy keygen can be used to generate account keys from a given password and is able to generate new passwords