rpc improvements

account
* function for reputation to score calculation
* Doku improved
amount
* better handling of offline mode
steem
* Doku improved
transactionbuilder
* handing of appbase for tx verify and broadcast improved
utils
* node list added as function
* not needed function removed
steemnoderpc
* handling of server errors improved
objects
* Amount fixed for appbase
exception
* RPCErrorDoRetry added, when a rpc call should be retried
grapherpc
* Handling of server error 500 to 511 added
* better handling when get_config does not work
unit tests
* nodes and nodes_appbase are take from utils
* keys is used instead of wif for Steem()
* unit test for broadcast added
* unit test for appbase for verify added
* test_operations added
* test_types added