[beanutils] Register converters by both target class and source class

One of the big shortcomings I see with the ConvertUtils is that you can't
intuitively register different converters based on source class. e.g.
SomeClass to String, and OtherClass to String. Especially since multiple
ConvertUtilsBeans can co-exist, it only makes sense to be able to get away from
depending on the toString() method for ALL conversions to a String type (this
is the nearest match that I can see to what I'm asking for).