ggnet submission -- a function for making a network plot from an object of class network or igraph, using ggplot2

https://github.com/briatte/ggnet
precision in example
example correction
gitignore
additional city-firms example
robust node.group argument
network package dependency
better example code
example correction
additional functions

functions used in a paper using website networks
little improvements

labels not drawn if all missing; function robust to igraph/network
namespace conflict
robust call to sna
final fixes to documentation

previous commits added a few functions, this one finalizes the function
for first release
support for NA values in node groups
tiny fixes
Merge pull request #20 from briatte/master

ggnet -- simple network plots with ggplot2