Security example for app.keys

[nbeguier]

Documentation:

• Add a security example for app.keys by redefining Keygrip to use sha512 instead of default sha1

[coveralls]

Coverage remained the same at 100.0% when pulling 494b0bb71b77e69c7256e6713d8ae9c8f08b6344 on nbeguier:security_example into f81d7130ff4fd44ad97bd0b807034bd46964db8a on koajs:master.