Add new module for flow based connectivity and cut algorithms
Fix doctests and python twoisms for the new connectivity module
Add Robins and Alexander bipartite clustering
Rename 'stack' to 'queue' in BFS for clarity
Add ancestor and descendant functions for DAGs

Ancestors are all nodes from which you can reach the source node.
Descendants are all nodes reachable from the source node. Both are
defined only in DAGs.
Make check for DAG optional in descendants algo

When searching for descendants many times, it is cumbersome to check
each time whether the graph is a DAG. Therefore, this check can now
be disabled by a keyword argument.
Improve performance of descendants and ancestors

The `descendants` function for DAGs was slow because it performed a
check of whether the input graph was a DAG. This is now off by default.

In addition, the inverse function, `ancestors`, would previously
reverse the entire graph to find ancestors using BFS. A modification
to the `bfs_edges()` function allows reversing the BFS itself, avoiding
the cost of the full graph reversal.
Refactor functions to use faster nx.shortest_path_length
Merge branch 'master' into jni-ancestors

Conflicts:
	networkx/algorithms/traversal/breadth_first_search.py

Resolve conflict introduced by #772.
Add new functions to reference documentation. #791
Add new clustering functions to the documentation, plus some small docstring improvements
Merge pull request #771 from jtorrents/bipartite-clustering

Add Robins and Alexander bipartite clustering
Fix header spacing for development docs.
Add spacing in pdf download links.
Merge pull request #813 from hagberg/doc-fix

Doc fixes
modified grid_graph generator to not mutate input parameters
Merge pull request #814 from AndrewWalker/master

More Pythonic behaviour for the grid_graph generator
Merge pull request #742 from jtorrents/connectivity

Add new module for flow based connectivity and cut algorithms
Add force scaling parameter to spring layout.

Add parameter k to specify optimal distance betweneen nodes in spring layout.
The default is 1/sqrt(number of nodes).
Rescale layout on every step to be in [0,1] box.
Remove spurious print and empty notes field.