Discovery

Fix #132 

- [x] Invalidate cache when ``collections`` parameter changes
- [x] Invalidate cache when the pair config or any related storage config changes
- [x] How should collections be *created*? The problem is that vdirsyncer currently raises an error if the local folder doesn't exist, but the default behavior was to create it. A new method has to be added to storages for creating collections.
- [x] Add discover-command
- [x] Revise collection parameter
- [x] Write docs for discovery (dedicated page)