Config Setup Tweaks

* API: Rest transport errors are now properly converted to SoftLayerAPIErrors
* CLI: Since all unknown exceptions have the traceback printed out, there's no reason to re-raise ValueError and KeyError in core.
* CLI: Adds timeout to `sl config setup` client
CLI: `sl config setup` defaults to public endpoint if no input is given
CLI: Retry username and api key/password prompts until non-empty in config setup
Merge pull request #137 from sudorandom/config_tweaks

Config Setup Tweaks