Prevent creation of FrameworkMethod or FrameworkField without underlying member.

The classes are relying on an underlying member. Fail early, because this makes it easier to find mistakes.
Override toString() with meaningful implementation.

The new implementation provides more useful information.
Change visibility of field fMethod to private.

There is a getter method for this field. Users should use this method.
Remove duplicate code.
Merge pull request #791 from stefanbirkner/member-improvement

Improve FrameworkMember, FrameworkMethod and FrameworkField