PNG sprite changes & new ascii mapping

Changes to the PNG sprite sheet and CSS in an attempt to fix the poor rendering in the latest release of Chrome.

• 1 pixel padding added in the sprite sheet
• CSS background-position changed from percentage to pixel offset
• CSS width and height changed to 64px
• removed CSS background-size
• removed unnecessary sprite SASS file

Updated ascii and PNG demos to reflect new changes.
Changed :) :-) :] =] =) mapping from 😄 to :slight_smile:
Bumped cache busting param.