Clearer explanation when exception has no message

"No exception supplied" is misleading; actually there is an exception,
but there's no message string.
Fixed documentation of disable_constraint_checking

The docstring and base implementation of disable_constraint_checking do not indicate that a return value is expected for proper behavior.
Added clarification to the docs, pointing out that unique_for_date only considers the date portion of DateTime fields.
Recommend using the bcrypt library instead of py-bcrypt

* py-bcrypt has not been updated in some time
* py-bcrypt does not support Python3
* py3k-bcrypt, a port of py-bcrypt to python3 is not compatible
  with Django
* bcrypt is supported on all versions of Python that Django
  supports
Merge pull request #1064 from Wilfred/master

Minor spelling fix
Merge pull request #1039 from cannona/master

Clarified what unique_for_date considers
Merge pull request #1062 from dstufft/switch-bcrypt-recommendations

Recommend using the bcrypt library instead of py-bcrypt
Fixed #19934 - Use of Pillow is now preferred over PIL.

This starts the deprecation period for PIL (support to end in 1.8).
Removed unicode literals from PIL compat.
Fixed #20165 - Updated testing example to use django.test.TestCase.

Thanks Lorin Hochstein.
Improved the timezone middleware example slightly.

This change avoids having the timezone leak from a request to the next.
Don't unregister OSMGeoAdmin, other tests rely on it.
Fixed previous commit. (Don't commit from DjangCon!)
Fixed #20413 - Respect Query.get_meta()
Merge pull request #1069 from mfogel/ticket_20413

Fixed #20413 - Respect Query.get_meta()
Fix bug introduced in contrib.gis in 74f3884ae0
Merge pull request #1070 from mfogel/ticket_20413

Fix bug introduced in contrib.gis in 74f3884ae0
small typo, example pk=27 but description pk=24
Make the example usage of import_by_path less confusing.

Using import_by_path to import import_by_path is a really odd use case
and makes the code example difficult to read.
Merge pull request #1072 from hrbonz/doc_serialize_typo

Fixed typo in serialization docs.