Previously, it was impossible to have a Table that has
None for a schema name when the "schema" parameter on
MetaData was set.  A new symbol sqlalchemy.schema.BLANK_SCHEMA
is added which indicates that the schema name should unconditionally
be set to None.  In particular, this value must be passed within
cross-schema foreign key reflection, so that a Table which
is in the "default" schema can be represented properly.

Fixes: #3716
Change-Id: I3d24f99c22cded206c5379fd32a225e74edb7a8e

Fixed bug where by ROW_NUMBER OVER clause applied for OFFSET
selects in SQL Server would inappropriately substitute a plain column
from the local statement that overlaps with a label name used by
the ORDER BY criteria of the statement.

Change-Id: Ic2500c886cbfc83a1ad5a2681783f008b9f23838
Fixes: #3711

Change-Id: I040b75ff3b4110e7e8b26442a4eb226ba8c26715
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/234

Pull request courtesy Stefan Urbanek.  fixes #1957

- make docs for isolation level more consistent between postgresql
and mysql
- move mysql autocommit tests

the new behavior of the autoincrement flag as per ref #3216

handled within visit_select(); this attribute was added in the
1.0 series to accommodate the subquery wrapping behavior of
SQL Server and Oracle while also working with positional
column targeting and no longer relying upon "key fallback"
in order to target columns in such a statement.  The IBM DB2
third-party dialect also has this use case, but its implementation
is using regular expressions to rewrite the textual SELECT only
and does not make use of a "wrapped" select at this time.
The logic no longer attempts to reconcile proxy set collections as
this was not deterministic, and instead assumes that the select()
and the wrapper select() match their columns postionally,
at least for the column positions they have in common,
so it is now very simple and safe.  fixes #3657.
- as a side effect of #3657 it was also revealed that the
strategy of calling upon a ResultProxy._getter was not
correctly calling into NoSuchColumnError when an expected
column was not present, and instead returned None up to
loading.instances() to produce NoneType failures; added
a raiseerr argument to _getter() which is called when we
aren't expecting None, fixes #3658.