Allow dialects to customize group by clause compilation

Allow dialects to customize group by clause compilation

Refactored :class:`.SQLCompiler` to expose a
:meth:`.SQLCompiler.group_by_clause` method similar to the
:meth:`.SQLCompiler.order_by_clause` and :meth:`.SQLCompiler.limit_clause`
methods, which can be overridden by dialects to customize how GROUP BY
renders.  Pull request courtesy Samuel Chou.

Change-Id: I0a7238e55032558c27a0c56a72907c7b883456f1
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/474
(cherry picked from commit 33fccc486111fc6b41eab651cc7325c83099ad45)