Rename methods Reflect#on(Class) and Reflect#on(String) to onClass() to avoid ambiguities

[lukaseder]

The current overloading of Reflect.on() is unfortunate in a few cases where people want to reflect on instances of String or Class, but accidentally bind to the more specific methods that do something quite different from Reflect#on(Object). See e.g. #55

Those methods shouldn't overload the base method, but have an entirely different name. The old methods will stay in place and will be deprecated to document this change.

[lukaseder]

While the change is quite intrusive (lots of deprecations in calling code), it is not backwards incompatible. We'll remove the deprecated methods in jOOR 1.0. When reflecting on String or Class instances, callers still have to cast the arguments in order to bind to the non-deprecated methods.