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. 

Nitpicking is great! 

![ihavenoideahwtaiamdoing](https://cloud.githubusercontent.com/assets/28390/4489418/cf229c0e-4a1a-11e4-9a4c-1640fd2df7de.gif)