Add ability to specify idbag (#415)

[jeff-hagen]

Closes #415

Adds the ability to specify a collection .AsIdBag(). Note that the NHibernate xml mapping model is a little odd:

• The column attribute on collection-id is always required even though a column element can technically be specified.
• The type attribute is required and must be a .net type (varchar will throw an exception)
• The idbag is identical to the bag mapping, with the 2 exceptions:
  • The idbag has a collection-id element
  • The idbag doesn't have one-to-many