Columns quoting proposal.

Related to https://github.com/rails/rails/issues/2601

This is only a proposal to support column quoting in the SELECT and ORDER
parts of the select statement.
It lacks of tests, correct exception generation and I don'y know what else! :)

I don't know either the full impact on activerecord, even if I am using
inside a rails project without problems (at the moment).

I committed it on a branch derived from the v2.2.1 tag, but I tried a merge on the master branch without conflicts.