Add a registry for Transformer implementations

Hi,

This is my first development JIRA for BU2 so apologies in case i m missing out on basics. Beanutils1 has a functionality wherein all the converters are registered and are called when conversion in value is required. This functionality is missing for BU2 project. I also saw that BeanUtils1 uses WeakFastHashMap for this, which seems like is having issues across architectures and did see few JIRA's on this. Wanted inputs if having a synchronized instance of WeakHashMap wrapped in the TransformerRegsitry class and providing methods to register, deregister, restoreDefault, lookup be desired?