Remove flow functions from the base NetworkX namespace.

Also renamed files in the flow package so modules and functions have
different names; we obey the informal module-name-must-be-unique rule.
Updated tests and docstrings. The later needs a bit more work to clearly
explain that flow functions have to be explicitly imported from the
flow package.
Adapt connectivity package to the removal of flow_funcs from namespace.
Do not use ford_fulkerson in max flow tests for large graphs.

The goal is to speed up a bit the maximum flow tests for large graphs.
The regular tests for maximum flow algorithms are enough for making sure
that ford_fulkerson is correct. Also reused residual network when possible.
Improve docstrings for flow functions.

Explain that these functions have to be explicitly imported
from the flow package.
Add docstrings to build_residual_network and add it to sphinix docs.
Add draft of 1.9 api changes for the flow package.

Based on the previous drafts posted at #1162 .
Simplify __init__.py for the flow package.
Cosmetic changes in maximum flow tests on 'large' graphs.
Add test for MultiGraph input.
Merge pull request #1173 from jtorrents/finish-refactor

Remove flow_funcs from the base namespace