Skip tests on python implementations without getrefcount()

PyPy is one of these.
Added test to verify copysize handling in copy_expert

Not an original psycopg2 bug but present in pure python implementation,
e.g. ctypes issue #25 and cffi issue #5.

https://github.com/mvantellingen/psycopg2-ctypes/issues/25
https://github.com/chtd/psycopg2cffi/pull/5
Added regression test for infinity parsing in dates

...and a couple of typos.
Fixed comparison of `Range` with non-range objects

Fixes ticket #164.

Patch from Chris Withers on master.
Added doc example to convert date.max to infinity

See issue #163.
Untrack the connection before closing to avoid possible double-free

From Gangadharan S.A. Fixes issue #166.
Fixed doc index entry for infinite dates
Fixed manifest to avoid spurious messages during install

It was a while we were getting::

  no previously-included directories found matching 'doc/src/_build'

at install time. Also avoid a couple of Sphinx build files from sdist
package.
Bump up version for release 2.5.1
Bump to next dev release number
Fixed titles in newsfile

The first title at the same level of the main releases helps getting a
more regular ToC in the docs main page.
Fixed pickling of exceptions with no pgerror/pgcode set

Fixes ticket #170.
Meaningful connection errors report a meaningful message

Fixes issue #173.
Document that using %, (, ) in the placeholder name is not supported
Document that named cursors don't raise an exception going out-of-bound

See issue #174 for the details.
Really test named cursors in test_scroll_named()
Use bytestrings in infinite dates adapter example

Returning strings would result in unclear tracebacks on Py3 (see ticket #163).
Typo fixes and improvements to new_array_type() docs
Check connection type in lobject init

Fixes ticket #187.
Fixed dealloc of lobject->smode

I put it by mistake into the in the self->conn checked block in the
previous commit.