Backport recent security fixes from 1.9

This is a backport of 93a135d to 1.7.x

Remove erroneous addition of **kwargs to ManyRelatedManager.add()

Merge pull request #3 from brodie/patch-1

Remove erroneous addition of **kwargs to ManyRelatedManager.add()

Allow Template Context to take another Context instance in #new

Fixes a bug when calling Context#flatten when creating a new context based on an existing context.

Allow Template Context to take another Context instance in #new

Fixes a bug when calling Context#flatten when creating a new context based on an existing context.

1.7 -> 1.9 Compatability -- use_debug_cursor to force_debug_cursor

Merge pull request #6 from atlassian/mbertrand/port-pr2968

Port PR 2968 renaming use_debug_cursor to force_debug_cursor

1.7 -> 1.9 Compatability -- add() **kwargs

This introduces **kwargs to the add() method of RelatedManager.