Allow users to create a meta description info

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As described on  http://googlewebmastercentral.blogspot.com/2007/09/improve-snippets-with-meta-description.html , google and others can use the information in the meta description in the short preview they give.

I suggest giving users the ability to enter this description to each post if they want to. As this will need a database change, I would supply a patch as soon as migrations are supported.

This feature might also be used by a twitter app (just an example, does not have to)