Define an user agent for scripts and crawlers

[augusto-herrmann]

Well behaved scripts and crawlers should always send a custom, non-default user agent. We should:

• [x] define an user agent string for the scripts and crawlers that update publicbodies.org,
• [x] change scripts to use it, and
• [x] add instructions for script / crawler developers on using the proper user agent.

[augusto-herrmann]

Based on some best practices and examples I've find, I propose

PublicBodiesBot (https://github.com/okfn/publicbodies)

Considerations:

• the Mozilla bit should probably be kept out of it, considering we're not emulating a browser
• no version number, as tracking version numbers on each individual update bot could prove difficult
• provide a link to where sysadmins could find more information – this repository would probably explain better than our main site, but I'm open to arguments to the contrary
• we could use a fragment to represent jurisdiction (e.g. br), but I'd rather keep it simple