Removed support of Python 2.5

All supported distributions (Ubuntu, Debian and CentOS soon) are shipped with Python 2.6 or Python 2.7.
Now, many libraries doesn't support Python 2.5 (such as "Routes" in our case), and we are using some modules (like the "json" one) only available since the version 2.6.

This pull request removes the support of Python 2.5 and adds two new checks: one in the installer and one in centralreport.py, before starting the daemon.

Please notice that CR is not fully compatible with Python 3. Many changes are needed, and they will be done in a next release.