Add iD editor

We're feeling pretty good about the state of iD with the beta1 release, and since then have done some work to make it embed nicely in the Rails port. It's at a state where wider community adoption and feedback would be welcome, so we'd like to kick off the process of making it available on osm.org.

The outstanding technical questions. @tomhughes, interested in your thoughts.

* What's the best way to handle version updates? Would you like us to check in build *.js files to the iD repository so that they can be pulled in with vendorer?
* iD does not support IE <9. P2 will need to be the fallback, and perhaps we should simply not offer iD at all to those browsers.