Add aggregate field to schema for keywords

[emodric]

Since eZ Tags is the first candidate extension for inclusion in one of the next eZ Publish Community Project builds (http://share.ez.no/blogs/community-project-board/taking-the-community-project-one-step-further-bundling-community-extensions) and it uses this field for simple tag suggestions, I'm creating the pull request for you to review.

It would simplify the installation of extension as there wouldn't be need to modify schema.xml by hand.

[ilukac]

I second that :)