Implement generalized settings

[Former user]

Reported by folke: As subsequent extensions of BibSonomy's core functionality require user specific settings, we need on the long run a settings 'framework' which is open to new settings.

One solution could be a (user,key)->value like schema.

Please contact Robert and Folke for discussing further details.

[Robert Jäschke]

The generalization of how we store settings is rather a long-term work which will not be done in the next months.