This commit combines two changes. First, it moves the module
`networkx.generators.hybrid` to `networkx.algorithms.hybrid` (and moves
the corresponding test modules as well). Second, it adds missing
documentation to the functions in that module.

Conflicts:
	doc/source/reference/api_2.0.rst
	doc/source/reference/credits.rst
	doc/source/reference/generators.rst

This commit reimplements `complete_bipartite_graph` using a generator
expression instead of two nested for loops. Also adds the `bipartite`
node attribute (that was missing, as per #1375).

I've also moved the `complete_bipartite_graph` function from `classic.py`
to `bipartite.py`. I think it's better to have all bipartite generators
together.

The function `complete_bipartite_graph` is used by other functions, such as
`wheel_graph`, which was abusing the interface by passing -1 as the second
argument of `complete_bipartite_graph` (for `wheel_graph(0)`) and hoping to
get an empty graph with zero nodes. This is also fixed.

Adds the chordal p-cycle graph, a mildly explicit algebraic construction
of a family of 3-regular expander graphs. Also, moves both the existing
expander graph generator function (for the Margulis-Gabber-Galil
expander) and the new chordal cycle graph function to a new module,
networkx.generators.expanders.