sqlalchemy/orm, sqlalchemy/event, sqlalchemy/testing

to get all flake8 passing

Removed ungrammatical apostrophes from documentation, replacing
"it's" with "its" where appropriate (but in a few cases with "it is"
when that read better).

While doing that, I also fixed a couple of minor typos etc.
as I noticed them.

- put list.clear() instrumentation under "if not py2k"

setslice of ``[0:0]`` correctly, which in particular could occur
when using ``insert(0, item)`` with the association proxy.  Due
to some quirk in Python collections, the issue was much more likely
with Python 3 rather than 2.  Also in 0.8.3, 0.7.11.
[ticket:2807]

- rework the event system so that event modules load after their
targets, dependencies are reversed
- create an improved strategy lookup system for the ORM
- rework the ORM to have very few import cycles
- move out "importlater" to just util.dependency
- other tricks to cross-populate modules in as clear a way as possible

proxies, they aren't overriding getattr() or setattr() at all.  so all the
hardcoded getattr()/setattr() is removed from collections.py.   Lots of these
getattr/setattr were against the attributeimpl and decorated functions
and don't seem like they'd ever be needed; for a user that needs special access
to a collection, we can evaulate that use case and add a single point of
"unwrapping", and probably add a hook for it via
InstrumentationManager so that the collection implementation isn't complicated
by it.