Add Names type to allow selecting elements by excluding others

Here's a first stab at the kind of convenience syntax I spoke about on the mailing list. Comments welcome.

This allows things like:
```julia
julia> reload("src/NamedArray.jl")
        n = NamedArray(rand(2,4))
       setnames!(n, ["one", "two"], 1)       
["one"=>1,"two"=>2]

julia> n[!Names(["one"]), Names(["1"])]
NamedArray{Float64,2}
names: two
1
dimnames: A	B

.14859425443510443
```

To save more typing, the type could be renamed to e.g. N.