Created new convert package.

This package will contain the code responsible for data type conversions.
The ConversionException class was moved into the new package.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1512794 13f79535-47bb-0310-9956-ffa450edef68
Added ConversionHandler interface.

This interface defines the basic facade of the data type conversion component.
An object implementing this interface is associated with each configuration
object. Requests for type conversions are delegated to this object.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1512795 13f79535-47bb-0310-9956-ffa450edef68