avoid dict comprehension for python 2.6

Avoid dict comprehension in Configuration.py for portability with Python 2.6.
Dict comprehension (PEP 274) are only implemented in Python >= 2.7.
add non-empty format field name for python 2.6

Python 2.6 does not accept empty field name for format(), it raises ValueError
exception with error 'zero length field name in format'. This commit adds field
names. New loop variable mac is introduced in order to respect the 80 cols
coding standard limit.
add installation instruction for CentOS/Fedora

Updated README file to add installation instruction for CentOS/Fedora and
probably most RPM based GNU/Linux distributions.

Downgraded the needed version of Python from 2.7 to 2.6, thanks to the lasts
portability commits.

Also corrected some typos.