Snap Sensitivity; Bar Drag

Added snap sensitivity (snap.sensitivity) - accepts decimal values between 0 & 3 inclusive; added bar-drag (!!); bug fix: get correct values at onSnap by sending them within setTimeout:0; cleaner: relying on offset values instead of style (type String); slight performance improvement with constraint checker mitigation; improved hard snap, esp. when startAt values are not at markers; better destroy method.