added more constructors for v4
changed String to AbstractString for 0.4
streamlining and suggestions per discussion
IPv4(Array) -> Array(IPv4)
switch to map() to preserve concrete type
commenting out IPv4(Vector) -> Vector(IPv4) until a decision can be made
Update socket.jl

Removed the comments since pending decision has apparently been made :)
Merge pull request #9346 from sbromberger/v4-constructors

V4 constructors (take 2)