Added SendInput()

If you mind the :

    // #include <WTypes.h>
    import "C"

the `(*MouseInput)(unsafe.Pointer(&input)).mi = oneInput.Mi` trick could be done by the user but with this commit the user can use SendInput easily with :

    package main

    import "github.com/AllenDang/w32"

    func main() {
    	w32.SendInput([]w32.INPUT{
    		w32.INPUT{Type: w32.INPUT_KEYBOARD, Ki: w32.KEYBDINPUT{WVk: 0x56}},
    		w32.INPUT{Type: w32.INPUT_KEYBOARD, Ki: w32.KEYBDINPUT{WVk: 0x57}},
    		w32.INPUT{Type: w32.INPUT_MOUSE, Mi: w32.MOUSEINPUT{Dx: 5, Dy: 5, DwFlags: w32.MOUSEEVENTF_MOVE}},
    	})
    }