Fixes argument number in docs

The selectable is the third argument to with_polymorphic.
Merge pull request #79 from nibrahim/master

Fixes argument number in docs
- Added a new keyword argument ``once=True`` to :func:`.event.listen`
and :func:`.event.listens_for`.  This is a convenience feature which
will wrap the given listener such that it is only invoked once.
- A warning is emitted if the :meth:`.MapperEvents.before_configured`
or :meth:`.MapperEvents.after_configured` events are applied to a
specific mapper or mapped class, as the events are only invoked
for the :class:`.Mapper` target at the general level.
add a string example for self-refenretial many-to-many
unitttest has a SkipTest also but pytest doesn't
honor it unless nose is imported too...
- Fixed bug in new :paramref:`.MetaData.naming_convention` feature
where the name of a check constraint making use of the
`"%(constraint_name)s"` token would get doubled up for the
constraint generated by a boolean or enum type, and overall
duplicate events would cause the `"%(constraint_name)s"` token
to keep compounding itself.
fixes #2991
:paramref:`.MetaData.naming_convention` feature will now also
apply to :class:`.CheckConstraint` objects that are associated
directly with a :class:`.Column` instead of just on the
:class:`.Table`.
- Added a new feature :func:`.schema.conv`, the purpose of which is to
mark a constraint name as already having had a naming convention applied.
This token will be used by Alembic migrations as of Alembic 0.6.4
in order to render constraints in migration scripts with names marked
as already having been subject to a naming convention.
re: #2991
Remove tables from metadata when autoload fails

If autoloading of a table fails, don't register it in a metadata
instance. It seems that the original behaviour was accidentally
changed in f6198d9abf453182f4b111e0579a7a4ef1614e79, restore it.

Closes issue #2988