Support unattended api purchases

[Michael Nelson]

The sca REST api should enable an oauth'd request to:

1) Check the pay prefs for the user, and
2) purchase a new p3a subscription via the API

as per the spec at:
https://docs.google.com/a/canonical.com/document/d/1i8g-55GwQbDwseRDsISo4C1GEPOBsHuihiUt7oBy498/edit

QA Notes:
You can test the api using one of the helpers in USC (on a branch with the updated client):
   $ bzr branch lp:~michael.nelson/software-center/update-sca-client qa-purchase-api
   $ cd qa-purchase-api/utils/
   $ SOFTWARE_CENTER_BUY_HOST=https://sc.staging.ubuntu.com ./piston-helpers/piston_generic_helper.py --output=text --needs-auth SoftwareCenterAgentAPI retrieve_payment_method

QA Status: Purchasing is working [1], but retrieving the payment method is not - we get a 500 because an old version of python-canonical-payment-client is installed on staging [2], which doesn't support the account_preferences call [3].

[1] https://pastebin.canonical.com/84090/
[2] https://pastebin.canonical.com/82429/
[3] https://pastebin.canonical.com/82428/