Downloader for derby databases

[martijn]

It would be nice to be able to select and update derby databases automatically from both Cytoscape and PathVisio?

This would consist of two components:

* A gui for listing available and installed version, and initiating a download * A central place where a table of available versions can be found, as well as the databases themselves.

The GUI should be done as a separate bridgedb module so that the same code can be used by PathVisio? and Cytoscape