Int vertex indexing behavior is surprising

[Futrell]

Following up on: https://groups.google.com/forum/#!topic/julia-users/G5zArVhEd3M

This error was very surprising:

julia> using Graphs

julia> g = graph([3, 4], Edge{Int}[])
Directed Graph (2 vertices, 0 edges)

julia> add_edge!(g, 3, 4)
ERROR: BoundsError()
 in add_edge! at /Users/canjo/.julia/v0.4/Graphs/src/graph.jl:86
 in add_edge! at /Users/canjo/.julia/v0.4/Graphs/src/graph.jl:98

julia> edges(g)
1-element Array{Edge{Int64},1}:
 edge [1]: 3 -- 4

From #114 it looks like this is expected behavior because Int indices must be consecutive.
But this was very unexpected to me, because for vertices of other types, I can use arbitrary values:

julia> gs = graph([:cat, :dog], Edge{Symbol}[])
Directed Graph (2 vertices, 0 edges)

julia> add_edge!(gs, :cat, :dog)
edge [1]: cat -- dog

julia> gf = graph([3.0, 4.0], Edge{Float64}[])
Directed Graph (2 vertices, 0 edges)

julia> add_edge!(gf, 3.0, 4.0)
edge [1]: 3.0 -- 4.0

The special behavior of Int vertices seems like it is bound to cause this issue to come up again and again for new users.

Despite throwing the BoundsError, it looks like add_edge! accomplished its work successfully. Can this issue be resolved by something as simple as not throwing the error?