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 a backwards compatible change.