API improvements and internal cleanup

• Combine SpringClock with looper
• Add SynchronousLooper for doing synchronous simulation
• Add SteppingLooper for doing step by step simulation
• pull in hamcrest for testing and update tests
• activateSpring on velocity or currentValue changed
• Springs with no tension continue to move when velocity is applied until friction causes them to stop if no end value is set.
• setCurrentValue now performs setAtRest by default with a new method signature added to support the uncommon use case of setting current value without setting at rest