Added lanes:forward, lanes:backward and lanes:both_ways to default tags

[henrik-wolf]

As discussed in issue #86, I added the parsing for the tags lanes:forward, lanes:backward and lanes:both_ways, to provide a clearer picture of the number of lanes actually present, without breaking the old behaviour of the lanes tag.

I added documentation as to how we parse all the default tags (this probably needs some love... I feel it might be clearer, if we just copy the whole source of every parsing function into the docs)

In addition, I added some code to used the new tags during weight adding, as described in point 2. Let me know what you think about it.