Add DFS over edges.
Add some additional comments.
Add import statement for doctest.
Fix doctest again.
Adapt print statements in examples for Python 3 compatibility
Merge pull request #1204 from ysitu/print

Adapt print statements in examples for Python 3 compatibility
Improve .travis.yml formatting
Remove Python 3 incompatible print statement
Merge pull request #1205 from michaelsproul/print-function-fix

Remove Python 3 incompatible print statement
corrected docstring

The docstring of connected_component_subgraphs() had one parameter (copy=True) listed as a Return.
Merge pull request #1207 from drevicko/patch-1

corrected docstring
Include IronPython in .travis.yml
Merge pull request #1209 from ysitu/ipy

Test with IronPython on Travis
Fix unreliable tests uncovered by IronPython
Make katz_centrality compatible with IronPython
Make add_nodes_from compatible with IronPython
Fix I/O compatibility with IronPython
Fix I/O compatibility with IronPython (cont'd)
Ensure that write_gexf and write_graphml output XML declaration
Reapply "correct minor typos in edgelist.py"

This reapplies commit d189147, which was inadvertently reverted in
commit 002d537.