to support dialect-level reflection options for all :class:`.Table`
objects reflected.

- Added a new dialect-level argument ``postgresql_ignore_search_path``;
this argument is accepted by both the :class:`.Table` constructor
as well as by the :meth:`.MetaData.reflect` method.  When in use
against Postgresql, a foreign-key referenced table which specifies
a remote schema name will retain that schema name even if the name
is present in the ``search_path``; the default behavior since 0.7.3
has been that schemas present in ``search_path`` would not be copied
to reflected :class:`.ForeignKey` objects.  The documentation has been
updated to describe in detail the behavior of the ``pg_get_constraintdef()``
function and how the ``postgresql_ignore_search_path`` feature essentially
determines if we will honor the schema qualification reported by
this function or not. [ticket:2922]

type such as "charset" and "collation".  While MySQL wants all character-
based CAST calls to use the CHAR type, we now create a real CHAR
object at CAST time and copy over all the parameters it has, so that
an expression like ``cast(x, mysql.TEXT(charset='utf8'))`` will
render ``CAST(t.col AS CHAR CHARACTER SET utf8)``.

- Added new "unicode returns" detection to the MySQL dialect and
to the default dialect system overall, such that any dialect
can add extra "tests" to the on-first-connect "does this DBAPI
return unicode directly?" detection. In this case, we are
adding a check specifically against the "utf8" encoding with
an explicit "utf8_bin" collation type (after checking that
this collation is available) to test for some buggy unicode
behavior observed with MySQLdb version 1.2.3.  While MySQLdb
has resolved this issue as of 1.2.4, the check here should
guard against regressions.  The change also allows the "unicode"
checks to log in the engine logs, which was not previously
the case. [ticket:2906]

- update a few exclusions to support current pymssql.  passes all of test_suite and dialect/mssql

the indexing.  this is for more natural operation.
- also add cast() to the JSON expression to complement astext. This integrates
the CAST call which will be needed frequently.  Part of [ticket:2687].
- it's a little unclear how more advanced unicode attribute-access is going to go,
some quick attempts at testing yielded strange error messages from psycopg2.
- do other cross linking as mentioned in [ticket:2687].

underscore.  Courtesy Treeve Jelbert. [ticket:2897]

Remove terminated connections from the pool.