Merge "Support IF EXISTS/IF NOT EXISTS for DDL constructs"

diff --cc lib/sqlalchemy/dialects/mysql/base.py
Simple merge

diff --cc lib/sqlalchemy/dialects/postgresql/base.py
Simple merge

diff --cc lib/sqlalchemy/dialects/sqlite/base.py
Simple merge

diff --cc lib/sqlalchemy/sql/compiler.py
Simple merge

diff --cc lib/sqlalchemy/sql/ddl.py
Simple merge

diff --cc lib/sqlalchemy/testing/requirements.py
Simple merge

diff --cc test/requirements.py
Simple merge