Extend @show: allow multiple args, return value of last

This patch extends the `@show` macro in two ways, to make it more handy in debugging:
* Multiple arguments are allowed, e.g.

        x, y = 3, 4
        @show x y

    prints

        x	= 3
        y	= 4

* The value of `@show` is the value of it's last argument. Using this, `@show` can be inserted pretty much anywhere in an expression to produce a printout, e.g.

        f(args...) = some_computation_based_on_args

    can be rewritten to print out its return value like

        f(args...) = @show some_computation_based_on_args

    instead of

        function f(args...)
            result = some_computation_based_on_args
            @show result
            result
        end

I haven't come up with a case where it makes much sense to use both features at the same time. If anyone thinks that the value of `@show` with several arguments should be something else, please let me know.