Support _ in update step property names

Allows us to specify fields like user_id or user__username when updating
steps.

Also update the documentation to be more correct and include an example.
Merge pull request #454 from infoxchange/fix-update-step

Support _ in update step property names
add the explicit documentation section README.md

As has been said in the Zen of Python:

`Explicit is better than implicit.`
Merge pull request #455 from luzfcb/patch-1

add explicit documentation section on README.md
temporarily not running the integration tests
releasing 0.2.20