Add the allow_add_remove parameter to the get_serializer method

`_resolve_model` should work with unicode strings

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.

Allow customising ChoiceField blank display value

fix pep8

Typo fix.

Fixes #1535 (HTML widget missing `id` attribute)

Updated failing test from #1575

Add colon to time zone offset in readable_datetime_formats

Help docs

Use help_text, verbose_name, editable attributes for related fields 

Mark strings in AuthTokenSerializer as translatable 

Help docs