beem/steem.py: calculate with absolute used_power values

Fixes #46

Scheduled weekly dependency update for week 27

CLI
* upvote and downvote fixed
Snapshot
* refactoring
* update_vote and build_vp_arrays added for showing vote power history
Examples
* account_vp_over_time added

* Fixes issue #45 by checking the timestamp of last_payout instead of net_rshares. With these changes, voting of unvoted posts/comments is possible.
* Refactoring of Transactionbuilder and adding of _use_condenser_api for defining the use of condenser api on broadcasting when using appbase nodes.. 

* get_data is now fast
* update_rewards added
* enable_rewards added to build
* build_curation_arrays added

Snapshot
* impoved and STEEM and SBD history added
* Documentation added
* get_data, reset, build functions added

fix SteemConnect TransactionBuilder example

Fixes #42

Add vit to known_formats

Snapshot added
* This class can be used to access old account states. At the moment, only changes in vests are tracked over time.
* example added for plotting SP over time

Steem
* last_node added and refresh of json_config data is forced on node switch

* Update nodes in Nodelist
* Disable seed.bitcoiner.me node

Relase nodes for 0.19.45 added

Example
* A cache performance evalution script was added to examples
Blockchain
* set_cache_auto_clean disabled

reworking account.estimate_virtual_op_num

Fixes #36. More reliable convergence and therefore less RPC calls.

Scheduled weekly dependency update for week 26