Make WAL sequence calculation compatible with PostgreSQL 9.3

With PostgreSQL 9.3 WAL files are written in a continuous stream,
rather than skipping the last 16MB segment every 4GB, meaning WAL
filenames may end in FF.
PostgreSQL version in backup.info file is an integer
Add support for restore target name (PostgreSQL 9.1+)
27: Addresses potential corruption of WAL xlog.db files.

In barman.lockfile.release() the file is unlinked (deleted). This effectively
nullifies any future attempts to lock the file by a blocking process by deleting
the open file table entry upon which the flock is based.

This commit removes the unlink and instead unlocks the file and then closes the file
descriptor leaving the lock file and open file table entry intact.
Update setup.py to support py.test and recent setuptools
Initial Python 3 support
Added stub of release notes
Updated README and man page
Bump version to 1.2.3
Update the ChangeLog file
Added TODO list