Add client warning when running command without -a supplied

Issue: https://github.com/deis/deis/issues/1846

Adds a check for the --app parameter.

I haven't written any python in over a decade, so feedback welcome.

How does the client know it is inside an 'app folder'? 
I can't see any reference apart from the --app param. 

we should also check `self._session.app` as well as that gives us the name of the app based on the git repo/base name of the directory.