fix error caused by wrong field call inside getindex of TargetIterator

Hi,

this is how I reproduce the error:

```Julia
julia> sg = simple_inclist(4)
Directed Graph (4 vertices, 0 edges)

julia> add_edge!(sg, 1, 2)

julia> sit = out_neighbors(vertices(sg)[2], sg)
TargetIterator{GenericIncidenceList{Int64,Edge{Int64},Range1{Int64},Array{Array{Edge{Int64},1},1}},Array{Edge{Int64},1}}(Directed Graph (4 vertices, 0 edges),[])

julia> getindex(sit, 1)
ERROR: type TargetIterator has no field edges
 in getindex at /Users/luca/.julia/v0.2/Graphs/src/common.jl:120
```