Add button for zoom and change step of zoom-slider tool

Increase minHeight by 200px (to 900px) to allow full visibility of zoom
  slider tool inside Bitmapper window.
Add buttons to control zoom, to allow for easier stepping (0.1).
Change min value of zoom slider tool to 0.1 (10% of original value),
  make step value of zoom slider finer to 0.1
BUG=https://github.com/jackhou-chromium/bitmapper/issues/32
R=tapted@chromium.org

Review URL: https://codereview.appspot.com/192850043