Added Dict constructor from tuple array

Right now, we can get an array of tuples from a Dictionary via `collect`, but there's no constructor from such an array.  This pull request remedies that.

```julia
julia> a = [("A",1),("B",2),("C",4)]
3-element Array{(ASCIIString,Int64),1}:
 ("A",1)
 ("B",2)
 ("C",4)

julia> Dict(a)
["B"=>2,"C"=>4,"A"=>1]

julia> typeof(ans)
Dict{ASCIIString,Int64} (constructor with 2 methods)
```