Add support for generating incremental streams when pushing volumes

This partially addresses #46 - but does not completely resolve it.

Added here is support for a new parameter to `Filesystem.reader` to specifying which snapshots the receiver has.  The implementation uses this to generate an incremental stream if possible.  Also added is `IFilesystem.snapshots` which can be used to get a value to pass for this parameter.

Work to glue this together with the actual push implementation will come in a follow-up.
I'm sorry @exarkun I didn't understand this :confused: Can you add an explanatory comment. Why not just:
```
others = set(others)
...
if snapshot in others:
```