Add timeout to requests in Shodan._request()

[goncalor]

I've noticed that when facing network problems (e.g. Wi-Fi connection dropped) Shodan may hang indefinitely without timing out. The code calling the Shodan library has no control over this and may be left hanging for a response.

According to the requests documentation requests should have a timeout:

Most requests to external servers should have a timeout attached, in case the server is not responding in a timely manner. By default, requests do not time out unless a timeout value is set explicitly. Without a timeout, your code may hang for minutes or more.

These changes define a REQUESTS_TIMEOUT constant which defines a connect and read timeout of 30 seconds each. These timeouts are applied to the _request() method of the Shodan class.