Snapshot versions of symbols in the library.  Can be used like:

        Parameters
        ----------
        snap_name : `str`
            name of the snapshot
        metadata : `dict`
            an optional dictionary of metadata to persist along with the symbol.
        skip_symbols : `collections.Iterable`
            optional symbols to be excluded from the snapshot
        versions: `dict`
            an optional dictionary of versions of the symbols to be snapshot

def snapshot(self, snap_name, metadata=None, skip_symbols=None, versions=None):
        """
        Snapshot versions of symbols in the library.  Can be used like:

        Parameters
        ----------
        snap_name : `str`
            name of the snapshot
        metadata : `dict`
            an optional dictionary of metadata to persist along with the symbol.
        skip_symbols : `collections.Iterable`
            optional symbols to be excluded from the snapshot
        versions: `dict`
            an optional dictionary of versions of the symbols to be snapshot
        """
        # Ensure the user doesn't insert duplicates
        snapshot = self._snapshots.find_one({'name': snap_name})
        if snapshot:
            raise DuplicateSnapshotException("Snapshot '%s' already exists." % snap_name)

        # Create a snapshot version document
        snapshot = {'_id': bson.ObjectId()}
        snapshot['name'] = snap_name
        snapshot['metadata'] = metadata

        skip_symbols = set() if skip_symbols is None else set(skip_symbols)

        if versions is None:
            versions = {sym: None for sym in set(self.list_symbols()) - skip_symbols}

        # Loop over, and snapshot all versions except those we've been asked to skip
        for sym in versions:
            try:
                sym = self._read_metadata(sym, read_preference=ReadPreference.PRIMARY, as_of=versions[sym])
                # Update the parents field of the version document
                mongo_retry(self._versions.update_one)({'_id': sym['_id']},
                                                       {'$addToSet': {'parent': snapshot['_id']}})
            except NoDataFoundException:
                # Version has been deleted, not included in the snapshot
                pass

        mongo_retry(self._snapshots.insert_one)(snapshot)