Allow API clients with the 'signin' permission.

Apps all implicitly support this permission, even if they don't have a
`SupportedPermission` object to say so. By changing the Rake task to use
the `supported_permission_strings` method, we can use this permission
when creating API clients.
Merge pull request #99 from alphagov/api-client-signin-permission

Allow API clients with the 'signin' permission.