Add optional number to releases command

Add a `-n`, `--num` flag to the `releases` index command, to specify the number
of releases to be shown.

Implements #494
Merge pull request #1012 from catsby/release_number

Add optional number to releases command
Merge branch 'master' into px-dynos
allow size to be an arbitrary word (string)

Leave the validation to the server side
API v3 returns the full size description (eg.: '1X', as opposed to '1').
ps:resize - add PX to the usage example
use APIv3 for ps:resize