Allow SQLAlchemySchemaNode subclassing

[arnaud-morvan]

As we want relationship's nodes ceated with the same type as parent node.

Here is an example of subclassing:

import colander
from colanderalchemy import SQLAlchemySchemaNode


@colander.deferred
def deferred_request(node, kw):
    return kw.get('request')


@colander.deferred
def deferred_dbsession(node, kw):
    return kw.get('dbsession')


class GeoFormSchemaNode(SQLAlchemySchemaNode):

    def __init__(self, *args, **kw):
        super().__init__(*args, **kw)
        self.request = deferred_request
        self.dbsession = deferred_dbsession