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. 

this isn't required any more with the changes on L108-110. the function will be called and will spew a warning on line 2049. Try the following:

```
$ deis apps | grep go
example-go
$ mkdir /tmp/example-go
$ cd /tmp/example-go
$ deis releases:list
!    WARNING: No app specified.
!    Run this command from an app folder or specify which app to use with --app APP.
!    WARNING: No app specified.
!    Run this command from an app folder or specify which app to use with --app APP.
```