started random walk graph
added dd model generator
Merge branch 'master' of https://github.com/wangz10/networkx
Merge remote-tracking branch 'upstream/master'
added DD model
added DD graph to __all__
added unit test for duplication_divergence_graph
cleaned up left over codes
modified the range of allowed retention probability
modified the range for retention probability to (0,1] in the exception message
fixed typo...
Merge branch 'master' of https://github.com/wangz10/networkx into dupdiv
PEP 8 and other minor code edits.
Test exceptions as well.
Add duplication-divergence generator to release notes.
Double backticks for source code in documentation.
A little more docstring cleanup for `duplication_divergence_graph`.
Maintain ticket order in release notes.
Merge branch 'master' of github.com:networkx/networkx

Conflicts:
	doc/source/reference/api_2.0.rst