add test for error message in eccentricity function

Changes the error message when an infinite path length is encountered so
that it is different based on whether the graph is directed or not.

Use sphinx stable branch via github on ReadTheDocs

The stable branch of sphinx has the one-line fix for autosummary.
So even though the release on pipy doesn't have it, we can get it from github.

Adds unit test for new use of string.format().

Use format() instead of modulus for node formatting in error message to treat tuples correctly.

Instead of :

```
'dict' objects are unhashableTraceback (most recent call last):
  File "<builtin>/app_main.py", line 75, in run_toplevel
  File "mk_graph_file.py", line 34, in <module>
    if d['weight'] > 3])
  File "/usr/local/lib/pypy2.7/dist-packages/networkx/classes/graph.py", line 1488, in subgraph
    for n in bunch:
  File "/usr/local/lib/pypy2.7/dist-packages/networkx/classes/graph.py", line 1815, in bunch_iter
    "Node %s in the sequence nbunch is not a valid node."%n)
TypeError: not all arguments converted during string formatting
```

You get:

```
'dict' objects are unhashableTraceback (most recent call last):
  File "<builtin>/app_main.py", line 75, in run_toplevel
  File "mk_graph_file.py", line 34, in <module>
    if d['weight'] > 3])
  File "/usr/local/lib/pypy2.7/dist-packages/networkx/classes/graph.py", line 1488, in subgraph
    for n in bunch:
  File "/usr/local/lib/pypy2.7/dist-packages/networkx/classes/graph.py", line 1815, in bunch_iter
    "Node {} in the sequence nbunch is not a valid node.".format(n))
NetworkXError: Node ('java', 'performance', {'weight': 5}) in the sequence nbunch is not a valid node.
```

Implement reciprocity for directed graphs.