Immutable store with serialization

The PNGJS3 now supports immutability through an Immer storage that can be serialized to a plain object that can be stored in a Redux storage.

The changes are:

• Immutable store (setters and getters should allow for the same API as before) - issue #49
• There is a shape attribute with the width and the height
• For storage you can now use serialize() and static deserialize()

First npm release

Switch to ES6 with babel-compilation
Additional functionality to retain original bitmap + more