[Snowy] OAuth in Snowy

[Stuart Langridge <stuart langridge canonical com>]

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OK, I've added OAuth support to Snowy. OAuth endpoints are at
/oauth/{request_token,authenticate,access_token} and everything under
api/ now requires OAuth (and does not work with basic auth). Two patches
attached.

I had to patch piston; the piston patches are required for OAuth support
to actually work, and are taken from
http://bitbucket.org/ephelon/django-piston/changeset/a7f81eae936c/ in
consultation with jespern (who hasn't yet merged these patches upstream).

Your comments invited. :-)

sil