Coherent return types for Kruskal vs Prim

See https://github.com/JuliaGraphs/SimpleWeightedGraphs.jl/issues/17

When applied on a `SimpleWeightedGraphs`, one function returns a vector of `Edge`, the other a vector of `SimpleWeightedEdge`

Related discussion on Graphs.jl:
- https://github.com/JuliaGraphs/Graphs.jl/pull/116
- https://github.com/JuliaGraphs/Graphs.jl/pull/125