handle models that use "email" as username_field

[imposeren]

If model have not 'usrername' field because User.USERNAME_FIELD is "email" then user creation fails. This PR should fix that issue

[imposeren]

Should fix #158

[codecov-io]

Codecov Report

Merging #157 into master will decrease coverage by 0.06%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master     #157      +/-   ##
==========================================
- Coverage   92.03%   91.96%   -0.07%     
==========================================
  Files          35       35              
  Lines        1256     1270      +14     
==========================================
+ Hits         1156     1168      +12     
- Misses        100      102       +2

Impacted Files	Coverage Δ
social_django/storage.py	98.46% <75%> (-1.54%)	⬇️
social_django/management/__init__.py	58.92% <0%> (+2.05%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 751b572...5c29df6. Read the comment docs.

[omab]

Thanks!