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've never seen the `:return <type>` syntax before. Is it valid? http://sphinx-doc.org/domains.html#info-field-lists