Increase SQLAlchemySchemaNode configurability and minor fixes

This pull request:

* Adds the ability for any keyword arguments to be passed to ``SQLAlchemySchemaNode`` during __init__ -- which in turn get passed to the SchemaNode __init__.  This allows ``SQLAlchemySchemaNode`` to accept all options that its parent class does.
* Allows for the configuration of the ``Colander.Mapping`` nodes that are produced (eg part of Sequence()/SQL relationships) by way of an attribute on the given mapped class (``__colanderalchemy__ = {'widget': ..., 'title': 'My Object'}``, for instance).  
* Fixes a bug preventing schema nodes that weren't mapped to an SQLAlchemy attribute from allowing the ``dictify()`` function to run.  Now, non-SQLAlchemy nodes are ignored with a debug message logged.  (Useful in the case of adding something like a CSRF or CAPTCHA field to a schema -- something that shouldn't be stored in the database, but is needed all the same)

I've noticed that the documentation for v0.2 of ColanderAlchemy is still that v0.1.  So, I'm more than happy to update the doco for the new iteration of ColanderAlchemy -- will this fine for inclusion?  If so, I'll get started on it and when I'm done, I'll send a pull request.