- Added new ENUM type to the Postgresql dialect, which exists as a schema-level

- Added new ENUM type to the Postgresql dialect, which exists as a schema-level
construct and extends the generic Enum type.  Automatically
associates itself with tables and their parent metadata
to issue the appropriate CREATE TYPE/DROP TYPE
commands as needed, supports unicode labels, supports
reflection.  [ticket:1511]

- MySQL ENUM now subclasses the new generic Enum type, and also handles
unicode values implicitly, if the given labelnames are unicode
objects.

- Added a new Enum generic type, currently supported on
Postgresql and MySQL.  Enum is a schema-aware object
to support databases which require specific DDL in
order to use enum or equivalent; in the case of PG
it handles the details of `CREATE TYPE`, and on
other databases without native enum support can
support generation of CHECK constraints.
[ticket:1109] [ticket:1511]

- types documentation updates

- some cleanup on schema/expression docs