Use get_by_natural_key() instead of get()

[jianyuan]

We use email instead of username as our natural key. The library should get the user using the value nominated by User.USERNAME_FIELD.

[fangli]

Good catch!
Since this method has backward compatibility issues for old django version, could you get a chance to add try-catch for this method?
So we try get_by_natural_key first, then fallback to get when specific exception happens.

[jianyuan]

@fangli Done!