HF20 - Add support and documentation for witness_set_properties

### Version of Python you are running ###
2.7.12


### Version of steem-python you are running ###
Latest


### Expected Behavior ###
To be able to use the new `witness_set_properties` once HF20 is live and find appropriate documentation on it's use.


### Actual Behavior ###
N/A


### Steps to reproduce ###
N/A


### Stack Trace ###
N/A
