Add IS (NOT) DISTINCT FROM operators

Add IS (NOT) DISTINCT FROM operators

None / True / False render as literals.
For SQLite, "IS" is used as SQLite lacks
"IS DISTINCT FROM" but its "IS" operator acts
this way for NULL.

Doctext-author: Mike Bayer <mike_mp@zzzcomputing.com>
Change-Id: I9227b81f7207b42627a0349d14d40b46aa756cce
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/248