Neutron Refactoring

v2_0 -> v2
No options (redesign)
More features (extension-related)
Some bugfixes
Mystery solved :)

What made me doubt is the fact that to deserialize the JSON, if there is no default constructor, the one with parameters must be used and Gson must know in which order the values must be passed. It is not an option to pass null values and set them afterward by reflection, because the constructor can have null check validations.

I've debugged the code, and Gson, when the `@ConstructorProperties` annotation is not found, picks an internal implementation that ends up using their [UnsafeAllocator](https://code.google.com/p/google-gson/source/browse/tags/gson-2.2.4/src/main/java/com/google/gson/internal/UnsafeAllocator.java), which uses `sun.misc.Unsafe` to directly allocate a new instance of the deserialization class in memory without calling the constructor. After this, it sets the field with reflection, according to the field annotations.

Although this works, it has several issues: the constructor might not be used at all and all validations in there may not be triggered, and also using `sun.misc.Unsafe` might not work in all implementations of the JVM. 

I think we should try to be as portable as possible, so I'd recommend adding the annotation, now we know more about the deserialization internals. WDYT?