Add-ons following Heroku's pattern

Support to commands:

* addons # list installed addons
* addons:add ADDON           #  install an addon
* addons:list                        #  list all available addons
* addons:remove ADDON1 [ADDON2 ...]  #  uninstall one or more addons

Request and responses documentation are here:  https://github.com/azukiapp/apiserver/wiki/Openruko-Addons. You can apply this page at openruko's wiki too after all.

More information:

One can **edit** `apiserver/bin/addaddon` to register an addon (it's a workaround and call the procedure for now). By default, test plan is created and the addon is available as 'beta' version.

[MySQL Provider](https://github.com/azukiapp/mysql-provider) is an example of provider that we used to test the addons.

These commands **are not implemented** (and someday will be):

* addons:docs ADDON                  #  open an addon's documentation in your browser
* addons:downgrade ADDON          #  downgrade an existing addon
* addons:open ADDON                  #  open an addon's dashboard in your browser
* addons:upgrade ADDON              #  upgrade an existing addon

All feedbacks are welcomed.