isolation_level settings for postgresql, mysql

(cherry picked from commit 49eb39366a0ca54ce973147802864fd7c359adb3)

(cherry picked from commit 07a199d929aa920b5b2b5d8b2d7008626a731f1f)

clarify MySQL unicode documentation, bringing it up-to-date
with current DBAPI support

operation with unicode parameters.  SQLAlchemy now passes both
the statement as well as the bound parameters as unicode
objects, as PyMySQL generally uses string interpolation
internally to produce the final statement, and in the case of
executemany does the "encode" step only on the final statement.
fixes #3337

(cherry picked from commit dcf5408f7d315b4d9ddec5d0d696eb364d763099)

Conflicts:
	lib/sqlalchemy/dialects/mysql/pymysql.py

(cherry picked from commit 00f37f3984c35fc4a2ee5687e798c5ca36953d6c)

levels; :meth:`.Connection.get_isolation_level`,
:attr:`.Connection.default_isolation_level`.
- enhance documentation inter-linkage between new accessors,
existing isolation_level parameters, as well as in
the dialect-level methods which should be fully covered
by Engine/Connection level APIs now.

(cherry picked from commit c3d898e8d06c7e549bb273fc8654f5d24fab2204)

format
- add an extra doc to MySQLdb
- changelog

(cherry picked from commit 182553b7409cfa6673483d03f29bc4c462336577)

(cherry picked from commit 0d15791a6e3ec35e3c2e297026d8396742e30a34)