Give a name to Sanic app

[wangqr]

Running python server.py will give a warning:

server.py:8: DeprecationWarning: Sanic(name=None) is deprecated and None value support for `name` will be removed in the next release. Please use Sanic(name='your_application_name') instead.
  app = Sanic()

So this PR gives a name to the Sanic app.