Implement construct_graph()

The `network` module implements graph analysis of mutual follower
relationships using NetworkX.

The first step is constructing a mutual follower graph, done here
in `construct_graph`. This function is inspired by example 4-10
of Matthew Russell's *Mining the Social Web*.
Implement build_clique_collection()

This function uses networkx to find cliques in the mutual follower
graph, then stores those cliques in a clique collection. The
schema of documents in the clique collection include:

- `members`: a list of `str_ids` of members of the clique
- `size`: the number of members in the clique

The clique collection is rebuilt everytime `build_clique_collection`
is called.

This is inspired by Example 4-11 of *Mining the Social Web*.
Implmement build --cliques command

This provides command line access for the clique collection
construction pipeline with `networkx` and the `network.py` module
in particular.

The `--clique-cname` flag sets the collection name to store
clique documents in.

Added `networkx` as a dependency in `setup.py`.

Updated README with clique info; this need to be bolstered with
a tutorial.

Next steps: analyze cliques and make clique data available to
the user.