Issue 802: Support external creation of IEdmModel

Adding the ability to generate EntitySetLinkBuilder and ActionLinkBuilder
when the model doesn't have one. With this change, one can build an
IEdmModel externally (without using the model builder) and use it without
any annotations. See sample here on how to build IEdmModel directly from
you EF DBContext - https://gist.github.com/raghuramn/5864013