Dropped PSYCOPG_EXTENSIONS flag

Building without extensions has been long broken and nobody really cares
about a pure-DBAPI implementation (which could be created using a wrapper
instead).
Dropped PSYCOPG_NEW_BOOLEAN flag

Introduced in 2.0 beta 8, 2006 A.D. Went absolutely untouched in 8 years
of refactoring, when Python 2.5 and PostgreSQL 8.1 roamed the earth.

I would say it has stood the test of the time.
Dropped HAVE_PQFREEMEM flag

It was necessary before PG 7.4, in versions which have long been unsupported.
Merge branch 'drop-flags'
Readme cleaned up and converted to reST

Dropped outdated info and references to the INSTALL, which is less
up-to-date than the docs.

Converted to reST to have it prettier on GitHub.
Added info about running the test suite in the docs

They were in the INSTALL file, which is quite out-of-date now.
Also fixed a couple of other things.
Dropped content from the INSTALL file

The docs have it right.
Dropped outdated HACKING file
Merge branch 'clean-text-files'
Fixed doc example about Error attributes

Catch the Error, not everything!

Also, whitespaces.
Move the `extras` module up in the docs

It is more interesting than `tz` and `pool`.
Take the PYTHONPATH into account when building the docs
Document we known 9.4 errorcodes
Self-updating copyright year in docs
Use virtualenv as build environment for the docs