initial work on SpookParams class. #26
added methods .setGlobal(Stiffness|Relaxation|EquationParameters) to World. Changed & added a few Equation properties. Removed properties .stiffness, .relaxation and .useGlobalEquationParameters from GSSolver
updated demos and tests according to new equation parameter related stuff
added .setStiffness and .setRelaxation methods to Constraint
exposed default Equation parameters through Equation.DEFAULT_STIFFNESS and .DEFAULT_RELAXATION
using the exposed default equation params in demo