Show relation type descriptions when choosing a relation type

Fixes issue #1862.

Taginfo has a [separate API](http://taginfo.openstreetmap.org/taginfo/apidoc#api_4_relation_wiki_pages) for getting information about a relation type.
![relationinfo](https://f.cloud.github.com/assets/1231218/1234749/3483a312-2989-11e3-9401-c765cb9df62d.png)