Add BRFares plugin

It doesn't do EVERYTHING in the ticket, but it's pretty close. It's
missing the ability to give a link to a textual version of the
restriction code (which would be VERY useful but I couldn't figure out
the best way to do it), and the ability to break fares up into groups if
there are too many of them. It has been tested a bit in #tim32.
Fix fares with missing adult/child fare

Some fares don't have an adult/child fare. I didn't realise this was the
case. Add checks for missing adult/child fare and give semi-ugly but
working output if so.
Fix the same bug but in a different place

The same bug was present in fares that don't have an adult fare (had
forgotten these existed) when listing fares.
Add URL to restrictions PDF

Add URL to official NRE restrictions PDF
Merge pull request #23 from Muzer/master

Add BRFares plugin, close #22