Switch to OAuth client credentials flow.

This removes the need to go via the joind.in site (via a WebView) to authenticate - it uses the username and password to obtain an access token, similar to the web2 site.

Sign-in page probably needs some kind of design work, screenshot as follows:

![device-2014-06-25-091746](https://cloud.githubusercontent.com/assets/231551/3382864/4e8881f2-fc41-11e3-8a79-cb9a0c6cfd21.png)

but the Accounts screen in Android now shows the correct username post-sign-in (retrieved via the user's URI):

![device-2014-06-25-091927](https://cloud.githubusercontent.com/assets/231551/3382874/7dc961d4-fc41-11e3-88c6-4a353f22ec14.png)

The Buildozer build config will need to be updated with the new client ID etc.