JSONB support once again, as they suddenly
switched on unconditional decoding of JSONB types in version 2.7.1.
Version detection now specifies 2.7.1 as where we should expect
the DBAPI to do json encoding for us.
fixes #3439

- changelog
- versionadded + reflink for new pg storage parameters doc
- pep8ing
- add additional tests to definitely check that the Index object
is created all the way with the opts we want
fixes #3455

features that other objects like :class:`.Index` now do, that
the column expression may be specified as an arbitrary SQL
expression such as :obj:`.cast` or :obj:`.text`.
fixes #3454

"max_row_buffer" execution option for BufferedRowResultProxy
- also add documentation, changelog and version notes
- rework the max_row_buffer argument to be interpreted from
the execution options upfront when the BufferedRowResultProxy
is first initialized.

psycopg2cffi dialect, in particular that the current 2.7.0 version
does not have native support for the JSONB type.  The version detection
for psycopg2 features has been tuned into a specific sub-version
for psycopg2cffi.  Additionally, test coverage has been enabled
for the full series of psycopg2 features under psycopg2cffi.
fixes #3439

Add support for specifying PostgreSQL index storage paramters (e.g.
fillfactor).