- Created StepFactoryBean, as a "master" factory that holds all the properties
configurable on a step and decides which type of step to create.
- the StepFactoryBean is used to generate a tasklet step as well as chunk-oriented step because if the 'tasklet=' attribute is specified on at one level and the <tasklet/> element is specified at another, the factory bean will be able to throw a useful error, whereas the parser would not know.
- removed hacks from TaskletStep and SimpleStepFactoryBean