Add initial support to OpenStreetMap accounts.

[sim6]

Related to #1503

[zbynekwinkler]

@sim6 Thanks for the contribution. We are currently focusing on infrastructure issues (no user visible changes now). The branch elsewhere contains refactorings of the code your pull request is based on but it is not done yet. I'd like to ask you to hold on to this until we are through this work and update your contribution then. You've picked most likely the worst possible time to contribute to the external accounts. Sorry about that :(

[sim6]

I can develop a support of OpenStreetMap accounts on top of elsewhere. But if you accept this pull request, then gittip can accept OpenStreetMap accounts right now, and offer a free[-as-in-speech] login option(#1503).

[chadwhitacre]

IRC

[chadwhitacre]

@sim6 is going to help us with #1369, then we'll circle back here. IRC

[chadwhitacre]

Moving this from WIP to Stalled.

[zbynekwinkler]

@whit537 Shouldn't we land this?

[chadwhitacre]

@zwn Yes! I moved it to stalled because no one is actively working on this. We should probably try to clear out stalled before we start new work.

[zbynekwinkler]

Closing in favor of #1848 (the same thing in a local branch).