Staging repositories

We should set up staging repositories (${ubunturelease}-staging), where new packages could be tested, and moved to Medibuntu main repositories (${ubunturelease}) if no bug has been reported within a week.

i.e :
* There's a new upstream release of googleearth. We package it for edgy, and put it in 'edgy-staging'. There is no bug report from testers within a week, so the package is moved to 'edgy'.
* There's a new upstream release of skype. We package it for dapper, and put it in 'dapper-staging'. A tester reports a bug for it. Fixed package is uploaded to 'dapper-staging'. There is no bug report from testers within a week, so the package is moved to 'dapper'.