Update topology module to support NetworkX 1.9

This commit casts the output of NetworkX function
connected_components_subgraphs to list to support the upcoming
release NetworkX 1.9. The reason for this commit is that between
release 1.8 and 1.9 the return type of this Networkx function
changed from list to a generator, breaking Icarus code.
Move SequenceNumber to util module
Add alpha property to TruncatedZipfDist class