Feat route relations

Dear imposm 2.5 maintainers,

I have worked on some modifications to imposm (version 2.5) in my branch at https://github.com/numenor/imposm/tree/feat-route-relations . I am aware of it not cleanly applying on your master currently (although I am a bit confused because of the history of master git is showing me; but more about that if you think my changes to be relevant).

I am wondering whether you are interested in the features implemented by me (and michalmacki https://bitbucket.org/michalmacki/imposm-routes/overview , on whose work mine is based), and whether it is worth the effort of sorting out the remaining issues (rebasing my branch on your current master, and probably removing one conflict where we have both done a similar change in postgis.py -> _stmt).

The features in short:
* support route relations
* support ":" characters in tag names

I have tested my branch on renderings with an own style, on an imposm import of the whole of Austria, and it seems fine. But I suggest that someone with better knowledge about the internals of imposm reviews my changes, because in mapping.py -> TagMapper.tag_filter_for_relations I now let all relations pass, as long as they have some key/value pairs requested by the mapping configuration (not only multipolygons/boundaries/land_area as originally, and not only routes which were my original motivation).

Best wishes, Holger Schöner (numenor at github, numenor@ancalime.de)