v0.4 release

Changes

• Switched from export default to module.exports to work in Node
• Flags/methods/executors are now optional
• Added immutable flag
• Added static .extend() method
• You can now pass init context by fluent(schema)(initContext)

BBackward compatibility

Backward compatibility is not broken but you can get unexpected behaviour if you accidentally pass smth to instance creator (how is this possible)