Simple format for value + derivative

[simonbyrne]

It would be great to be able to do something like

val, drv = ForwardDiff.value_derivative(f, x)

[simonbyrne]

also a simple Newton solver would be handy as well

[asprionj]

There's several (simple) ways of constructing the val, drv = ... syntax yourself. But for me, this is more about performance rather than nice syntax, see #401. The value is anyways calculated, so please return it as well, without any overhead.