Use the Python heap for conn->dsn
Fixed build on windows

Will fail with error: initializer element is not constant
Testing boilerplate unified in a single base class

The class makes a connection always available, allows creating
new connection and closes everything on tear down.
Added cursor_factory connection attribute and connect() parameter
Enable by default the range typecasters

Otherwise it would have been enabled importing psycopg2.extras,
which is a surprising behaviour.
Dropped __all__ from modules

They were only used to generate docs with Epydoc, now largely forgotten.

Imports in extras cleaned up to expose the API only.