Add "description" and "keywordCache" to Plugin

The `description` attribute will be used to show the plugin brief. It's
intermediate between the name and the readme.
The `keywordCache` attribute is needed to perform a query against the
plugin keywords. This is the only way to achieve this, mainly because
limitations on Waterline ORM.
Refactor action descriptions. Add a "find" example
Fix the update operations on a Plugin