provision: add Ready method to the app type

This method will set the app as Ready.
testing: add Ready method to FakeApp

Related to #450.
app: added Ready method to App

It will change State to "ready" and update the app in the database.
app: add "ready" key in the JSON representing the app

Related to #450.
cmd/tsuru-base: don't count units that aren't named in app-list
cmd/tsuru-base: add new column to app-list output

This column indicates whether the app is ready or not for deployment.

Related to #450.