Add exact bandwidth sizes into rpc-method show.ratio

I added exact sizes of bytes into the "show.ratio" methods of json-rpc and xml-rpc interface.

This conforms to many other methods were a exact sice and a preformatted size value is returned. I tried to keep it backwards-compatible and didn't rename the keys "up" and "down".

With the exact values it should be possible for a client to calculate a differential value by calling it every n seconds and display something like "120 kBit/s".

I'm a python guy and this are indeed my first lines of c++-code. Please tell if this is not the correct way of doing it. It compiled and worked for me :)