extract step wrap to <StepDict> STEP_REGISTRY load method
use lambda expression instead of function wrap in step decorator
added tests used to make StepDict refactoring
added StepDict.load_func to load a step from a single function
make step decorator usable without call on it thanks to StepDict.load_func, and document it
added StepDict.load_steps to load steps from an object methods
added decorator '@steps' to load a list of step from class methods