Changed incorrect class name in pagination docs

Remove the 'docs for 3.0' banners.

Update documentation over PAGE_SIZE

Fixed missing word in CursorPagination docs

Support serializing unsaved models with related fields.

This replaces all locations of the old setting `DEFAULT_PAGE_SIZE` for the current setting `PAGE_SIZE`

Remove `MergeDict`

- In both ManyRelatedField, provide an empty return when trying to
  access a relation field if the instance in question has no PK (so
  likely hasn't been inserted yet)
- Add relevant tests
- Without these changes, exceptions would be raised when trying to
  serialize the uncreated models as it is impossible to query
  relations without a PK
- Add test to ensure RelatedField does not regress as currently 
  supports being serialized with and unsaved model

Fix customizing field mappings link

The class MergeDict is deprecated and will be removed in Django 1.9

CreateOnlyDefault call default.set_context() only on create

Updated CreateOnlyDefault to call set_context on its default

fix DictKey initial value

Documentation test fix double word

Ensure validators are new-style classes on python2

Dont swallow errors in callable sources

Fix UniqueTogetherValidator for NULL values

List resource not updated between requests