add a __str__ implementation to APIException

Add a __str__ implementation to rest_framework.exceptions.APIException. This helps for logging raised exceptions. Thanks.
Merge pull request #1501 from jacobg/patch-1

add a __str__ implementation to APIException
replace page with page_size to avoide confusion
Merge pull request #1510 from uploadcare/docs-page-vs-page-limit

replace page with page_size to avoide confusion
Fixed comment.
Merge pull request #1511 from maurodec/master

Fixed comment.
Fix python syntax in filtering docs
Merge pull request #1514 from craigatron/master

Fix python syntax in filtering docs
Adds test that blank option is added when required=False on RelatedFields
Merge branch 'refs/heads/master' into #1408
Skip new test for Django < 1.6
Merge pull request #1515 from carltongibson/#1408

Added test for #1408 — Already Passes
Fix dict_keys equality test for python 3.
Merge pull request #1523 from sigilion/master

Fix dict_keys equality test for python 3.