[offlineimap] XOAUTH2 implementation (#254)

[Nicolas Sebrecht]

Documentation looks well written but:
* I don't understand why it stands in its own repository example. We already have the `RemoteExample` repository section for this.
* `auth_mechanisms` configuration option is missing XOAUTH2.

Here's how to make a dependency optional:
```
if configuration.option == True: # enabled
    import blah
    #code using blah
```

Updating a git topic should be done in its own branch so that once updates are done you can squash the changes in the correct (already written) commits while keeping the previous history clean.

Thanks for the good work. ,-)

---
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https://github.com/OfflineIMAP/offlineimap/pull/254#issuecomment-144991774
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