allow use of a custom token model

[BrunoVillanova]

This commit allow the use of a custom Token model for expiring token.
Eg:

class CustomModelObtainExpiringAuthToken(ObtainExpiringAuthToken):
    model =CustomModelToken

obtain_expiring_auth_token = CustomModelObtainExpiringAuthToken.as_view()

[coveralls]

Coverage remained the same at 100.0% when pulling dee4cf5 on BrunoVillanova:master into e62f1f9 on JamesRitchie:master.

[coveralls]

Coverage remained the same at 100.0% when pulling dee4cf5 on BrunoVillanova:master into e62f1f9 on JamesRitchie:master.