Contributors mark and osm.org/contributors

*This is a larger change, an RFC on [talk] is pending.*

This pull request adds a new page under `openstreetmap.org/contributors` like this:

![Screen Shot 2013-01-10 at 2 56 21 PM](http://cl.ly/image/0e1j06211R0Z/Screen%20Shot%202013-01-11%20at%209.06.35%20AM.png)

It is linked from a contributor's mark on the front page map:

![Screen Shot 2013-01-10 at 2 59 42 PM](https://f.cloud.github.com/assets/108094/57981/4a3571c4-5b60-11e2-8a47-9e4a0513cc87.png)

The idea behind this contributor's mark and this new page is to provide a visually compelling way to link back to OpenStreetMap from maps and communicate clearly what OpenStreetMap is about: community built, open geographic data, anybody can contribute.

Ideally this contributor mark finds broad adoption and is used by OSM data users [like this](http://cl.ly/image/1O452w1V2r3Y).

The contributor's page can be configured to: 

- Reflect custom tile sets
- Reflect the specific OSM data user ("Foursquare uses maps powered by data from OSM...")
- Set an unobtrusive link to OSM service provider ("Maps from MapBox")

To help other users customize openstreetmap.org/contributors & use the link icon on their maps, there is a link in the bottom left corner of openstreetmap.org/contribors that brings up a little 'attribution builder' wizard that looks like this:

Step 1:
![Screen Shot 2013-01-10 at 3 03 39 PM](https://f.cloud.github.com/assets/108094/57999/d5b8980c-5b60-11e2-90f3-db88f9675e66.png)

Step 2:
![Screen Shot 2013-01-10 at 3 01 58 PM](https://f.cloud.github.com/assets/108094/58003/de52b466-5b60-11e2-8c27-829020d3759a.png)

That code snippet is for a contributors mark meant to be dropped in the bottom right corner of maps, which links back to the /contributors page.

Aside from a pending review phase, this pull request still needs:

- Translatable strings on osm.org/contributors
- Model releases and permissions from photographers
- Reconciliation with osm.org/copyright

@lxbarth will follow up momentarily with a corresponding RFC on [talk] in a bit elaborating on this idea.

Original design is by @yhahn. You can explore example usages of the contributors mark here (click on the mark on each of the examples and review how osm.org/contributors changes):

http://yhahn.github.com/byosm/examples.html