Fix SIF export to make it possible to export from a pathway imported as network

[khanspers]

See cytoscape/cytoscape-manual#51

Pathways include gpml objects in the node table that are not real nodes, and therefor don't have a name. This breaks the SIF export. The export should work, at least for pathways imported as networks. Even these networks include anchor nodes with no name. A simple fix could be to add a name to all nodes to make the SIF export work.