Warn on non-Connection present and accommodate for Engine

Warn on non-Connection present and accommodate for Engine

A warning is emitted when an object that's not a
:class:`~sqlalchemy.engine.Connection` is passed to
:meth:`.EnvironmentContext.configure`.  For the case of a
:class:`~sqlalchemy.engine.Engine` passed, the check for "in transaction"
introduced in version 0.9.0 has been relaxed to work in the case of an
attribute error, as some users appear to be passing an
:class:`~sqlalchemy.engine.Engine` and not a
:class:`~sqlalchemy.engine.Connection`.

Change-Id: I95ef38955c00511d3055362a03284fb91677595f
Fixes: #419