several optimiations

* add account get_notifications and mark_notifications_as_read
* beempy notifications has been added
* bridge api support added
* config storage improved and add get_default_config_storage, get_default_key_storage and get_default_token_storage
* list_all_subscriptions and get_account_posts added
* image upload url fixed for HIVE
* reduce number of performed api calls on Steem object creation