Don't set logging level when used as a library

[milliams]

Server.serve calls Server._setup_logging which explicitly sets the logging level to INFO and hooks up the tornado logger to the output too.

As a user of a library, I should be able to be in control about what gets printed to the screen of my app and what dependencies of mine log to where.

I can fix this by monkey-patching the code with:

server = Server()
server._setup_logging = lambda: None
server.serve(root=".")

but I think that by default the library should not alter the loggers and they should instead be set in cli.py, allowing the CLI experience to be nice but giving library users the flexibility to set things how they wish.

[pradyunsg]

I think we should use our own logger instance instead of the global logger for this. PRs welcome! ^>^