Enhance support for InstanceCreator

```
It would be great if gson supported the notion a global InstanceCreator, one
which is delegated to for the construction of all types. API could be something
along the lines of:

GsonBuilder.registerInstanceCreator(InstanceCreator)

This would allow easier delegation to Guice, Spring, etc., for the construction
of all objects to be populated by gson as opposed to registering one
InstanceCreator per type, as is currently required.

Alternatively, the registerTypeHierarchyAdapter method could be enhanced to
support instance creators, such that a "global" InstanceCreator could
effectively be registered as one that creates instances of base type
Object.class.
```

Original issue reported on code.google.com by `pacesysj...@gmail.com` on 8 Apr 2012 at 5:20