Allow override/extension of BeanAdapter.coerce() using handler/delegate interface

Due to the issue I've reported, :
Re: https://issues.apache.org/jira/browse/PIVOT-704

For Pivot 2.1,
I may suggest a handler/delegate interface for BeanAdapter to allow
developers to override the handling BeanAdapter.coerce() after
exhausting all of its if-else logic.

Re: http://apache-pivot-users.399431.n3.nabble.com/JSONSerializer-can-t-deserialize-to-BigDecimal-td2464302.html