Customizable schema generator classes

Add Django 1.11 into *.md and setup.py

Clarify get_schema_fields signature

Bumped django-guardian version.

I had to look for the signature of `get_schema_fields()`in source code.

Updated monthly report link on funding page

remove django 1.11 from travis allow_failure matrix

django 1.11 to stable release in tox

Update existing vary headers in response instead of overwriting them.

Update links after moving to encode org

Previously, any existing vary headers would simply be wiped out by DRF. Using patch_vary_headers assures that existing headers remain.

Pass initkwargs stored on view to instance

Use overridden settings exception handler

Instead of using the api_settings exception handler, we use the
overridden settings attribute to find the correct handler.

Closes #5054

fix whitespace in code example

Update Boolean field to support 'yes' and 'no' values

Python Reference:
distutils.util.strtobool(val)
Convert a string representation of truth to true (1) or false (0).
True values are y, yes, t, true, on and 1; false values are n, no, f, false, off and 0. Raises ValueError if val is anything else.

Fix parser names in docstring.

Fix typo at doc string

fix unique=True validation for ChoiceField

Add drfpasswordless to Third Party Packages and Authentication pages

Document adding django_filters to INSTALLED_APPS