Addresses several points in issue #6.
Stripped out secondary canvas for mouse-over popups.
Sprites are now partitioned into pages of 50x50 pixel grids.
Each 50x50 grid is displayed as a page that the user can
  switch between using a drop-down.
Pixel grids are now subdivided into groups of 10x10 pixels.
Interface in index.html is now wrapped in <table> instead of <div>.
JavaScript alerts when user uploads file of unsupported type (PNG or GIF).
JavaScript alerts when user uploads image file that is too big.
Image processor tracks number of pixels of each RGB value, including
  clustered values in the tree.