Updated docs to reflect new tests layout.

Thanks to Ramiro Morales for the initial patch.
Ensured that we have an absolute path to RUNTESTS_DIR.

This is needed to make 'python runtests.py' work, otherwise only
'python ./runtests.py' would work.
fixed admin_scripts tests on python 3.3
Changed test_runner tests to use full python path for settings files.
Refactored qs.add_q() and utils/tree.py

The sql/query.py add_q method did a lot of where/having tree hacking to
get complex queries to work correctly. Refactored the logic so that it
should be simpler to understand. The new logic should also produce
leaner trees.

The changes cascade somewhat, as some other parts of Django (like
add_filter() and WhereNode) expect boolean trees in certain format or
they fail to work. So to fix the add_q() one must fix utils/tree.py,
some things in add_filter(), WhereNode and so on.

This commit also fixed add_filter to see negate clauses up the path.
A query like .exclude(Q(reversefk__in=a_list)) didn't work similarly to
.filter(~Q(reversefk__in=a_list)). The reason for this is that only
the immediate parent negate clauses were seen by add_filter, and thus a
tree like AND: (NOT AND: (AND: condition)) will not be handled
correctly, as there is one intermediary AND node in the tree. The
example tree is generated by .exclude(~Q(reversefk__in=a_list)).

The added tests are for lookup resolving to aggregates in some complex
cases, and to negated aggregate lookups.

Fixed #17600, fixed #13198, fixed #17025, fixed #17000, fixed #11293.