Set version to "next".

This indicates a "volatile" versions for all commits between actual
releases (which are tagged).
Add small graph generators ('generators/small')

The following graph generators are added:
  - jsnx.generators.small.make_small_graph (jsnx.make_small_graph)
  - jsnx.generators.small.bull_gaph (jsnx.bull_graph)
  - jsnx.generators.small.krackhardt_kite_graph
    (jsnx.krackhardt_kite_graph)
Add 'algorithms/centrality/betweenness'.

The following methods are added:

  - jsnx.algorithms.centrality.betweenness.betweenness_centrality
    (jsnx.betweenness_centrality)
  - jsnx.algorithms.centrality.betweenness.edge_betweenness_centrality
    (jsnx.edge_betweenness_centrality)