Build: Upgrade to Grunt 0.4 API

* Upgrades to grunt 0.4.5
* Upgrades to grunt-jquery-content 2.0.0
* Removes many grunt plugins which are now bundled into grunt-jquery-content
* Removes many unused and unnecessary tasks
* Removes directory from sample config since there's only one useful value

Requires updating WordPress plugin from grunt-wordpress 1.0.7 to
gilded-wordpress 1.0.0.
README: Update URLs for required WordPress plugins

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