[GH-298] by sandermarechal: Pagination using SQL walkers

This issue is created automatically through a Github pull request on behalf of sandermarechal:

Url: https://github.com/doctrine/doctrine2/pull/298

Message:

A CountSqlWalker and LimitSubquerySqlWalker have been implemented. By
default the Paginator will use these SQL walkers. When a query already
uses custom SQL walkers, the Paginator will fall back to the existing
TreeWalker implementations. Improvements:

• Support for more complex DQL queries using named mixed results with
  GROUP BY and HAVING. For example:

SELECT g, u, COUNT(u.id) AS userCount
FROM Entity\Group g LEFT JOIN g.users u
GROUP BY g.id
HAVING userCount > 0

• Support for entities with composite primary keys in the CountSqlWalker
  and LimitSubquerySqlWalker. Only the WhereInWalker still needs to be
  updated for full composite primary key support. But someone smarter
  than me needs to look at that and figure out how to build a WHERE IN
  query that can select rows based on multiple columns.