Fixed "sudo -n" and "sudo -v" for OS X Mavericks

CentralReport must have administrative privileges to install or uninstall itself.
In the bash scripts (install.sh and uninstall.sh), we are testing whether the sudo session still alive. On OS X 10.9 Mavericks, the command "sudo -n" return 0, even if the sudo session is not alive.

Replaced by "sudo -v -p", with a custom message.
If the sudo session is alive, the message is not displayed.

![capture decran 2013-10-30 a 23 21 02](https://f.cloud.github.com/assets/1638158/1441803/9a38dbc4-41b1-11e3-9ca0-a96e0b751e2d.png)