Add support for Singletons

[therealmitchconnors]

Singletons provide a way to represent a single item at the service root. They live in the EntityContainer tag, just like EntitySets, Actions, and Functions. I would be willing to add support if you would be willing to merge my changes...

[tuomur]

PRs are welcome :)

[tpow]

It appears that @tuomur added singleton support, probably because PR #15 gave him motivation. This issue can be closed.

Tim