Moved methods in a seperate helper class.

These "convert" methods were static methods, so I think they
belong in a helper class. They should not overload the "AbstractArrow"
class instead.
I solved compatibility issues with "switch" statements
(Java 6 does not support switch statements with Strings)
Redirected to new "ArrowHelper" class.

I forgot to use the ArrowHelper class instead of the AbstractArrow class for
the new "converter" methods (from arrow to integer to string, and so on)
Deleted unnecessary "Pfeile" directory.