Make OrderedSet constructor work on iterables, like base

[kmsquire]

For v0.3, the Set interface has been changed, so that Sets are now constructed from iterables. OrderedSet should make the same change.

(Need to check if this is true for Dicts as well.)

julia> a = [1,2,3]
3-element Array{Int64,1}:
 1
 2
 3

julia> Set(a)
Set{Int64}({2,3,1})

julia> OrderedSet(a)
OrderedSet{Array{Int64,1}}([1,2,3])

This last one should be:

julia> OrderedSet(a)
OrderedSet{Int64}(1,2,3)

Assigning myself, but I may not get to it soon, so anyone else should feel free to change this. It should be an easy change.