Set the volume always.

[wmarkow]

This is a proposed solution for #26. The setVolume() method sets the volume always but still caches the saved value.

I wanted to get rid that curvol cache value, the returned value could be read from the chip directly, but it needs to be converted back (by a map function for example) to a range from 0 to 100. There was a side effect, when I did a small test like:

player.setVolume(70);
uint8_t readBack = player.getVolume();

then the readBack variable was set to 69. Probably this is an effect of rounding when using map method. I gave up with this solution, so in my PR the curvol is not totaly removed.