Implement jumpTo method

This implements a jumpTo method to jump to a specific position of the song.
Example:

```javascript
spotify.jumpTo(15, function() {
  console.log('Jumped 15th second of the song');
});
```

The test is not optimal now as we need a timeout to give spotify some time
to catch up from the beforeEach "playTrack", but I wanted to keep this change
small.
Allow to set absolute volume

This allows to set the absolute volume of spotify. Note that sometimes you'll
run into a off-by-one error as spotify returns a volume that is often one higher
than the one you actually set. For example:

```javascript
spotify.setVolume(42, function() {
  spotify.getState(function(err, state) {
    console.log(state.volume); // will output: 41
  });
});
```

This shouldn't have any noticable effect for the user of the library though.
Add jumpTo documentation to the README
Add documentation for setVolume
Merge pull request #5 from rmehner/jump_to

Implement jumpTo method
merge rmehner-set_volume
use syntax highlighting for documentation