Multiple Repositories

From my point of view, this can be merged now. When the multipe repos feature is not used, the client should behave as previously, so no risk of breakage for normal users should be small.

Things to do:
- [x] really add / remove  repositories
- [x] store repositories in the json file
- [x] download ckans from repositories
- [x] install/upgrade/remove ckans that come from "other" repositories
- [ ] adapt the GUI to multiple repositories

Things that can be enhanced:
- possibly ordering of repositories (when multiple ones provide the same CKAN)
- and many other things I forgot :-)