fix(client): return non-zero on errors

Some commands were inconsistent and returned 0 when an error
occurred. This makes the client more consistent with errors.

closes #767
refactor(client): remove provider functions

Deis no longer handles provider code, so these functions should not
exist in the client.
feat(client): add auth command

It's handy to have a `deis help auth` command which will show off the
documentation for the auth commands available to the user.
Merge pull request #898 from deis/client-cleanup

refactor(client): client cleanup; return non-zero on errors