Tool shed publishing support.

 - New shed_upload command that can tar up a repository and publish it to various tool sheds.
 - Introduce global config file ~/.planemo.yml with optional tool sheds API keys, tool shed username, etc...
 - New config_init command to setup a template for this global config file.
 - Allow each repo to contain an optional .shed.yml describing name of repository (will fill this out with more options).

Warning: This depends on very new tool shed features that have not even been merged into galaxy-central yet.