find_cliques function

Only minimally tested. Needs top-level comment crediting NetworkX, and
needs a @graph_requires line
Export find_cliques function
Minimal tests for find_cliques
find_cliques: test for algorithm requirements

 - @graph_requires vertex_list and adjacency_list
 - graph must be undirected

See
http://networkx.lanl.gov/reference/generated/networkx.algorithms.clique.find_cliques.html#networkx.algorithms.clique.find_cliques,
the documentation for the code on which this is based, which specifies
that it is unsuitable for directed graphs.
Document find_cliques
Rename `find_cliques` to `maximal_cliques`.

This is for two reasons:

 - Other functions such as `connected_components` are named for what
   they return rather than what they do.
 - The function returns maximal cliques rather than all cliques.
Add docstring to maximal_cliques.

This credits the NetworkX find_cliques function, on which this code is
apparently
(https://github.com/pozorvlak/Graphs.jl/commit/4473d01bc3df181300ecf89f2e0169e46206e9d8)
based.