Improvements to identity handling and NH ISession extension methods

Adds new extension methods to an ISession in order to load entities by their
identity.  These may or may not remain in the API, I am currently just toying
with the ideas.

Also adds new parsing methods for identity instances.
Fixes missing interface member to get root mapping from a serializer

This commit adds a new 'root mapping' property to all key value serializers.
It was previously missing from the public interface.
Adds extension method for string dictionaries, creates a NameValueCollection
Fixes mapping of entity-types within the key/value serializer

This issue was creating problems when dealing with interfaces for the entities.
This is now fixed and unit tests written.
Refactor interface for the key/value serializer, adds new non-generic interface

The key/value serializer did not previously have a non-generic interface.  This
commit adds one.
Adds a few unit tests for the key value serializer

These tests were actually added in order to solve a problem in an unrelated
project, trying to diagnose whether or not there is a bug with the serializer.

It turned out that there was no bug in the serializer but these tests may
remain, they pass.
Additional test for the serializer, fixes a bug in the IEntity interface

The test exposes a bug that was discovered when deserializing entity instances
using the generic IEntity interface (or a derived type).  The problem was that
the Id attribute was not settable and thus would not deserialize properly.

Whilst the test is associated with the serializer, it really exposed a problem
with the entity namespace & related types.

The problem is now fixed.
Adds a unit test whilst trying to narrow down a problem.

In fact there was no issue with the key value serializer.
Adds new feature to dispose-and-replace a service.

This allows a single service instance to be disposed of and replaced using the
registered service factory.
Bump version numbers
Merge branch 'release-2.1.0' into production