Introduce API to Check if client is behind Tor or not

[fpietrosanti]

This ticket is to implement an API that can be queried by HTTP client (also with CORS) to know if he is behind Tor or not.

This API should use the cached information of onionoo coming from #10 implementation .

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[hellais]

The API should implement a JSONP (CORS headers are not required) interface that will allow the requester to obtain if their publicly exposed IP address appears to be from the Tor network or not.

The API should return at minimum the following JSON dict:

{
    'tor': true | false
        if the IP address appears to be coming from the Tor network

    'ip': string
         the ipv4 dotted quad or ipv6 address

    'country': string
         (optional) two letter country code from which the request has originated
}

[fpietrosanti]

Didn't we received some criticism about the risks related to JSONP, being
able to inject JS code into the client's browser (maybe there was a thread
on tor-talk)?

Wouldn't be safer something different than jsonp?