Implement OpenSRP OAuth username change

[moshthepitt]

v2 of OpenSRP has changed the username field from userName to username. We need to make this change here, in a backwards compatible way.

[moshthepitt]

 ~/superset-patchup git tag
v0.1.0
v0.1.1
v0.1.2
v0.1.3
v0.1.4
v0.1.5
v0.1.6

This change is immediately needed for Reveal Stage and Reveal Zambia which are both on Superset 0.29.0rc7. My worry is I am not sure if the latest version of Ketchup is compatible with that version of Superset.

@ukanga I see that for Hello Mwana (which is essentially the same version of Superset) you used a branch fix-redirect-v0.1.6 - should we tag a release off of this branch?

[ukanga]

@moshthepitt Yes, you can tag a release from that branch.

[ukanga]

As to the access of the username

...
username = user_object.get("username") or user_object.get('userName').
...

[moshthepitt]

Fixed by #49 and #52