updated tutorial for django 1.9

Docs: Fix repetitive word in the tutorial

Fix typo in relations docs

Create issue and pull request templates

Closes #3952: Remove old django checks from tests and compat

Closes #3937. Support callable as the value of `initial` for any `serializer.Field`

Just like regular Django fields, the example adopted uses a `datetiume.date.today` to
illustrate the functionality.

Remove skipping of tests for Django<1.8. Remove several functions from compat.py not needed for Django>=1.8

fix typo

fix typo

fix typo

fix typo

Add an usage example on how to pass a callable to the `Field.initial` in the docs.

Drop EOL Django 1.7

Fixed _force_text_recursive typo

Add missing space in serializer error message

Remove version string from admin template

Remove version string from base template

Fixes vertical html layout for BooleanField

`HTMLFormRenderer` renders `BooleanField` field as not checked if vertical layout are used.

Displayed warnings during test suite.

Silenced deprecation warnings on Django 1.8.

Use 'password123' as password for new users in the tutorial

Trying to create a new user on Django 1.9.x with the password 'password'
will fail with the error message 'This password is too common.'. Because
of that it is necessary to change the used password from 'password' to
'password123'.

Fixes docstrings in fields.py