v0.12.0

• Introducing signal.setHeader(key, value) to set a response header #61
• Introducing signal.getHeader(key) to return a response header #61
• Introducing signal.getRequestHeader(key) to return a request header
  #61
• Introducing signal.error(key) to return an error with a key #61
• Introducing signal.sendFile(path) to end a response with a file #69
• Introducing $.error('message') and $.error('stack') to allow better
  Error Exceptions handling with app.error(callback)
• Added example for signal.error
• Added example for signal.sendFile
• No more Object.observe errors. It has been replaced by the ES6 Proxy function
• Bug Fixes