EdgeList does not support undirected graphs

[littlebenlittle]

Looking at issue 35, it appears the tacit convention is to take the graph type as an argument. However EdgeList only creates directed graphs as can be seen at line 39.

[fabmazz]

I think the fact that this information is not written in the documentation is a serious problem, I started using this method and luckily I realized that the graph was wrongly read. The annoying thing is that the edges count is the same in the original and the re-read graph because of the way Graphs.jl stores the edges.

Maybe one could add an option to the Format specifier, so that an user can specify to read as a directed or indirected input graph