allow pure number textboxes

`@manipulate` only recognized Strings as text boxes, with this PR it also knows what to do with numbers.

````julia
@manipulate for μ in -5:5, σ in 0:5, min = -10, max = 10 
    dist = Normal(μ, σ)
    plot(x -> pdf(dist,x), min, max) 
end
````

edit: rebased on master