prepare release v0.3.0 (graphframes#138)

Prepare release 0.3:

update README, versions
update API doc in CC
hide protected methods in Logging

* prepare release v0.3.0
* do not mention python 3

Merge pull request graphframes#103 from thunterdb/95-update

[95] Official release numbers

Merge pull request graphframes#35 from thunterdb/1602-error

typo in test