Added Constraint("name").Exists() to SchemaTable syntax

I've added the Constaint Exist functionality to the schematable syntax. The underlaying code in the processors was already present, just not implemented yet on the syntax interface.

The solution file is changed as well due to usage of VS2013; I seem to be unable to exclude this file from the pull request. So you might want to skip that one, or how can I skip this file in the pull request? :)