Node merging

![screen shot 2013-10-02 at 7 51 28 pm](https://f.cloud.github.com/assets/58878/1259856/29a0ddc0-2bfa-11e3-9abe-28b30e134f62.png)

![screen shot 2013-10-04 at 10 30 52 am](https://f.cloud.github.com/assets/58878/1271363/c7143cde-2d1a-11e3-92f4-425f930633bd.png)

- [x] Button UI
- [x] Button data attributes
- [x] Filter relevant tags only
- [x] Delete osm_id in OSMChange (a bit hairy)
- [x] tag conflict modal
- [x] append relevant tags to existing tags
- [x] removeLayer when done
- [x] preserve the ability to reset