Refactor RecurrenceRule class

[dmfs]

The RecurrenceRule class contains about 1500 lines of code (not counting blank lines, JavaDoc and comments), about 30% of the entire library. It really needs to be split into multiple classes. That way we also gain more flexibility.
First candidates are:

• create dedicated Iterator classes and deprecate the iterator methods
• externalize serialization to String, this way we can provide other options, e.g. regarding inclusion of default values (Interval not set if it is one #95)

Moving the enums to top level classes would be a breaking change that needs to be be considered carefully.