Implemented looping audio

When one audio sample is paused in order to play another one the pause
time is returned allowing the next file to start at the point the
previous one finished.
Implemented class 'Changed' on .rateslider

The class changed is now attached to each rate slider when they are
first moved. A request to go on to the next page of the test is not
permitted until all sliders have been moved.
Vertical locked sliders

Rate sliders are now in the vertical orientation and are locked unless
the stimulus they correspond to is being played.
Moved reference buttons and added slider values

Slider values have been added to the page which update when the slider
is moved. Reference and mixture (for testing source separation
algorithms) buttons have been added.
Stylesheet tweaks

Tweaked style sheet in accordance with the new style.
Cosmetic tweaks