Refactor connectivity package.

Use the new algorithms and the new interfaces provided by the flow package.
The code is quite clearer, and the new algorithms and interface provide
a significant speed up in running time. For some problems, this code is one order
of magnitude faster (see #1102 for bechmarks). So far the code is still
backwards compatible, but we should discuss some possible non backward
compatible improvements.
Add cutoff test for local connectivity functions.
Increase test coverage in cuts module.
Fix error in node var names in minimum_node_cut.
Improve auxiliary digraphs for connectivity functions.

1. The node mapping needed for node connectivity and minimum node cuts
is now a graph attribute of the auxiliary digraph. Thus there is no
need for a mapping parameter in the local version of these functions.

2. Change the parameter name for the auxuliary digraph from `aux_digraph`
to `auxiliary`. Also be consistent on the auxiliary digraph variable name
in the code. Now it is always `H`.

3. Added small sanity check for the auxiliary digraph for node connectivity.
If a digraph is passed as a paramater for reuse, check that it has a graph
attribute with the node mapping. If not we raise instead of rebuild the
auxiliary digraph.
Added examples reusing data structures and improved docstrings.
Remove all_pairs_node_connectivity_matrix function.

With the addition of the example of how to compute local node connectivity
among all pairs of nodes reusing the data structures (added in the previous
commit in the docstrings of local_node_connectivity). We can remove this
function (which is the only one that has numpy dependency in the
connectivity package).
Improve cutoff handling: node connectivity is bounded by minimum degree.

This change imporves significantly the speed of node_connectivity in
denser graphs. For very sparse ones increases speed by ~5%.
Remove local connectivity/cut functions from the base namespace.

This change is backwards incompatible. Updated docstrings for all affected
functions with example usage. The global functions provide a good enough
interface for most uses of connectivity algorithms. More sophisticated uses
require explicit imports from the flow package anyway.
Merge 'master' to get the Travis CI fix.
Improve selection of node with minimum degree in cut functions.
Avoid deep copy and always use iterators for building auxiliary digraphs.
Do not use G.to_directed() to avoid unnecessary deep copy.
Added checks for only source or only target specified.

If so we raise an exception. Also added tests for all connectivity
an cuts interface functions to cover this situation.
Fix test for edge_connectivity missing target.

It was testing node_connectivity twice. Now we have a 100% test coverage
in the connectivity package.
Do not allow weighted computations in minimum_st_edge_cut.

All connectivity and cut algorithms are supposed to work on unit
capacity networks. minimum_st_edge_cut was the only exception, because
it uses the new interface to flow algorithms, is potentially able to compute
weighted cutsets. However, this complicated the implementation and could
result in a NetworkXUnbounded exception if an user used as argument a
graph with some but not all edges with an attribute called capacity. Since
minimum_cut computes weighted cuts, there is no need to duplicate
functionality here.
Improve generation of auxiliary digraph for edge connectivity.

Always build the auxiliary network and simplify the process of
building of the auxiliary digraph.
Clean up docstrings.
Revert "Remove all_pairs_node_connectivity_matrix function."

This reverts commit 7cb64b682355912c02ecc550e4cd3b1c119cd4ac.
Remove obsolete tests for all_pairs_connectivity_matrix.