Adds a generator for the chordal p-cycle graph.

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.
Merge pull request #1306 from jfinkels/expanders

Adds a generator for the chordal p-cycle graph.
fix waxman_graph domain

fixes #1302
change encoding in expanders.py to utf-8
Merge pull request #1309 from hagberg/waxman-domain

fix waxman_graph domain
Merge pull request #1308 from hagberg/expander-encoding

change encoding in expanders.py to utf-8
Allows parallel edges when converting from numpy.

Adds the `parallel` keyword argument to the from_numpy_matrix() and
from_scipy_sparse_matrix() functions. If the adjacency matrix has
integer entries, this is set to `True`, and the `create_using` keyword
argument is an instance of `MultiGraph`, then each positive integer
entry in the adjacency matrix is interpreted as the number of parallel
edges, each with weight 1, joining two vertices. Otherwise, if any of
these conditions is not met, the function behaves as it previously did:
an integer entry is interpreted as the weight of a single edge joining
the two vertices.

This is not a backwards compatible change.