Added a check for mbstring on install
Refactored upgrade process

Instead of duplicating knowledge between the core schema
and the update scripts... we now have a fixed base schema
and run update scripts on top of that.
Bumped version numbers
Support 'inactive' sources

Usefull to deprecate a source: can't be imported anymore but
stil available for rendering.
Deprecated Digg, Buzz, Qik and Seesmic
Fixed Vimeo imports