DropEnumType class available from postgres dialect
Public inspector method to load enum list

Provide opportunity to get enums list via an inspector instance public
interface.
- public method name is get_enums()
- return a list of dicts like other methods do
- don't combine 'schema' with 'name', leave them separate
- support '*' argument so that we can retrieve cross-schema
if needed
- remove "conn" argument
- use bound parameters for 'schema' in SQL
- order by schema, name, label
- adapt _load_enums changes to column reflection
- changelog
- module docs for get_enums()
- add drop of enums to --dropfirst
Merge branch 'pr126'

Conflicts:
	doc/build/changelog/changelog_10.rst
flake8 cleanup
- rename _InspectionAttr to InspectionAttr
- The :meth:`.InspectionAttr.info` collection is now moved down to
:class:`.InspectionAttr`, where in addition to being available
on all :class:`.MapperProperty` objects, it is also now available
on hybrid properties, association proxies, when accessed via
:attr:`.Mapper.all_orm_descriptors`.
fixes #2971
- The ``info`` parameter has been added to the constructor for
:class:`.SynonymProperty` and :class:`.ComparableProperty`.
- The ``info`` parameter has been added as a constructor argument
to all schema constructs including :class:`.MetaData`,
:class:`.Index`, :class:`.ForeignKey`, :class:`.ForeignKeyConstraint`,
:class:`.UniqueConstraint`, :class:`.PrimaryKeyConstraint`,
:class:`.CheckConstraint`.

fixes #2963
- repair against use here
- updates
on second thought we need to prioritize what really needs to be
here and what's just in changelog
pep8
- Removing (or adding) an event listener at the same time that the event
is being run itself, either from inside the listener or from a
concurrent thread, now raises a RuntimeError, as the collection used is
now an instance of ``colletions.deque()`` and does not support changes
while being iterated.  Previously, a plain Python list was used where
removal from inside the event itself would produce silent failures.
fixes #3163
- The string keys that are used to determine the columns impacted
for an INSERT or UPDATE are now sorted when they contribute towards
the "compiled cache" cache key.   These keys were previously not
deterministically ordered, meaning the same statement could be
cached multiple times on equivalent keys, costing both in terms of
memory as well as performance.
fixes #3165
pep8
pep8
- UPDATE statements can now be batched within an ORM flush
into more performant executemany() call, similarly to how INSERT
statements can be batched; this will be invoked within flush
to the degree that subsequent UPDATE statements for the
same mapping and table involve the identical columns within the
VALUES clause, as well as that no VALUES-level SQL expressions
are embedded.
- some other inlinings within persistence.py
- modify how class state is tracked here as it seems like things
are a little more crazy under xdist mode
- other test fixes
- don't add the parent attach event within _on_table_attach
if we already have a table; this prevents reentrant calls and
we aren't supporting columns/etc being moved around between different parents