Dynamic thumbnails on pager

Added dynamic thumbnail creation to pager.

responsiveSlides now accepts thumb and thumbSrc options.
Thumb can be bg or img depending how thumbnail will be showed on pager.
Thumb option requires pager to be set true.

Updated demo to reflect usage.

Added options:
 "thumb" : false,          // String: Show a thumbnail in pager. Options: false, img = as image, bg = as background-image
  "thumbSrc" : false,       // String: Define a thumb src as a dataset (data-thumb="src" is called thumbSrc : 'thumb' ). Defaults to false (not set)