making measurements work with Printf

[longemen3000]

in the work of making Optim work with measurements (and fixing #100 ), i found that the show error is due to a measurement numbers being incapable of being formatted by @sprintf macro. a stopgap solution is to use Printf.tofloat(x::Measurement) = tofloat(value(x)) but it would be great to apply the formatting to both the value and the uncertainty, any idea on how to do that?