Added vertical mode, upper and lower track images, thumb and track dimensions

[ericrisler]

NMRangeSlider (erisler additions)

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• Added "vertical mode" to slider to render and operate vertically.
  NOTE: because 0,0 is the upper left in iOS, the control is rendered
  upside down to the way you would expect it to function. This means that
  the handle at the top actually represents the "lower" value and if you
  slide it up, the lower value will decrease toward 0 (the opposite being
  true for the "upper" handle). For now (until myself or someone else
  changes the code) you will have to convert the upper and lower values
  after reading them. I've included this code in the example to
  illustrate.
• Added track and track background width property so you can explicitly
  set both. These values will only take effect if the track and
  background images have insets on them.
• Added track and track background padding property so you can inset
  the track from the end of the control's bounds.
• Updated default theme images and added new vertical theme
• Added lower and upper track image properties. These currently only
  apply if you enable the double slider (lowerHandleHidden==NO &&
  upperHandleHidden==NO). Note - at this time the "upper" track in
  vertical mode is in fact the bottom track. This is because (0,0) is at
  the top.
• Updated demo project showing new features.
• Added thumb width+height properties. If you set insets for the thumb
  images you can resize them using these properties.