Body.sleepTime uses Infinities + can create sleeping Body

[thygrrr]

RigidBodies can now be created InActive via optional boolean parameter to World.CreateRigidBody(...), which defaults to true, the former behaviour.

RigidBodies track their sleep time using float.PositiveInfinity when deactivated, ensuring they really never wake up on their own.

Unit Test Cases added in ActivationTests.cs