Attribution, usage limits and referrer requirements for Nominatim search

[ahocevar]

For the location search, we need to comply to the Nominatim Terms of Use, especially these three bullet points:

• No heavy uses (an absolute maximum of 1 request per second).
• Provide a valid HTTP Referer or User-Agent identifying the application (stock User-Agents as set by http libraries will not do).
• Clearly display attribution as suitable for your medium.

I think it will be best to implement this when #232 is merged.