Rename SteemWebsocket to NodeWebsocket and SteemNodeRPC to NodeRPC

Remove non appbase nodes and related tests from all unit tests Improve blockchain.stream() for appbase

CLI
* updatenodes added, this command can be used to update the nodes list
NodeList
* update_nodes added, this command reads the metadata from fullnodeupdate, which contain newest nodes information
* add option wss and https fo get_nodes
Unit tests
* use updatenodes in all tests

NodeList
* replaces get_node_list from utils.py
* is a list and contains all nodes as dict
* get_nodes() returns the url in a list
Utils
* get_node_list removed
Unit Tests and Examples adapted to the changes

Wallet
* getKeysForAccount added
* getOwnerKeysForAccount, getActiveKeysForAccount, getPostingKeysForAccount added
beemapi
* WorkingNodeMissing is raised when no working node could be found
GrapheneRPC
* cycle([urls]) is replaced by the nodes class
Nodes
* Node handling and management of url and error_counts is performed by the nodes class
* sleep_and_check_retries is moved to the nodes class
* Websocket, steemnodrpc were adpapted to the changes
Unit tests
* new tests for the nodes class
* tests adapted for websocket and rpcutils

CLI
* improved password handling and confirmation for changewalletpassphrase, delkey and createwallet
unit tests
improved test_cli
removed uneeded test for test_websocket

Steemnoderpc
* self.errror_cnt_call used instead of local variable doRetryCount
Graphenerpc
* num_retries_call added for setting a max number of retries on a rpc call
* self.n_urls added for getting the number of given nodes
* sleep on retry removed, when switching over to the next node
self.error_cnt_call used for counting rpc call retries
rpcutils
* sleep_and_check_retries improved
Unit tests
num_retries added
test_golos reduced

rpcutils.py added
Timestamp fixed for py27
New unit tests for test_rpcutils

Bug fix for account
Block structure changed, is alwas {'block': , 'id'} now.
block function removed, Block is used now
Bug fix in comment
Discussions_by_payout removed
Several bugfixes in witness
More exception added to steemnoderpc
Unit tests for appbase nodes with parameterized added
steemnoderpc-unittest added

Wallet contains steem instance

Python2 comp
* str comp added
* py23 with py23_bytes added with unit tests
* moved back to __bytes__
* super() made compatible with python 2
Account:
* allow and disallow moved from steem
* small improvements
Comment
get_votes added
Steem
* Doku improved
Vote
* bug fixes
* printAsTable
Wallet
* getPostingKeyForAccount added
test_vote added
test_testnet added
test_py23 added

some small bug fixes