fields: allow help_text on SerializerMethodField

...by passing through any extra *args and **kwargs
to the parent constructor.

Previously one couldn't assign help_text to a
SerializerMethodField during construction.
Merge pull request #1594 from jcsp/wip-methodfield

fields: allow help_text on SerializerMethodField
Fix _resolve_model to work with unicode strings
Merge pull request #1596 from pipermerriam/piper/suport_unicode_model_references

`_resolve_model` should work with unicode strings
Add the allow_add_remove parameter to the get_serializer method
Merge pull request #1603 from khamaileon/master

Add the allow_add_remove parameter to the get_serializer method
Note on configuring TokenAuthentication
Always use specified content type in APIRequestFactory

If `content_type` is specified in the `APIRequestFactory`, always
include it in the request, even if data is empty.
Merge pull request #1608 from dbrgn/test_client_contenttype

Always use specified content type in APIRequestFactory
Fix typo in docs
Merge pull request #1612 from rhcarvalho/patch-3

Fix typo in docs
Replace ChoiceField type_label
Merge pull request #1619 from khamaileon/master

Replace ChoiceField type_label
Fixed #1624 (thanks @abraithwaite)
Fix #1614 - Corrected reference to serializers.CharField
Merge pull request #1632 from fletchowns/master

Fix #1614 - Corrected reference to serializers.CharField