Issue #6 : Custom JsonSerializer and JsonDeserializer

It is now possible to add custom serializer and deserializer.
It just requires the user to create a class that extends AbstractConfiguration and add it into a gwt property.
The generator will parse the user configuration and use the defined serializer/deserializer when it encounters the type.
Issue #7 : Support for Guava
Merge remote-tracking branch 'origin/master'