Use flask's server by default
Use flask's server by default

Using this server in production is discouraged, but to implement a
proper asynchronous server with tornado/gevent every blocking call would
have to be converted to a non-blocking call.

Failing to do so causes deadlocks like senpy/senpy#35

For now, it is easier to just use the default server.

gsi-upm/senpy,gsi-upm/senpy,gsi-upm/senpy