Nested mappers cannot access session.parent

[jackqu7]

session.parent can only be accessed in a collection: field.Collection(field.Nested(SomeMapper))

it will be null in the case of a simple field.Nested(SomeMapper)

[mikeywaites]

This is required as part of the #114 Feature. We will need to allow Nested to access the parent mapper so that it can find the role being used to marshal the mapper session.