feaas/auth: add decorator for requiring authentication on views

Related to #27.
tests: fix get implementation for test_auth
feaas/api: use basic authentication for some routes in the api

Only the plugin is not authenticated, all the other endpoints are now
authenticated using basic authentication. tsuru will use the id of the
service and the password, both provided in the manifest. The API will
use these data as environment variables.

Closes #27.