0.7.2

• support horizontal scrolling with shift + wheel, for mouse with vertical wheel only
• add scrollbar option for optional overlay scrollbars
• fix bug: Math.max.apply over @_data gives NaN once @_data becomes sparse, which
  happens after scrolling far away. this made select all produce a NaN column bound.
• fix bug: select all bound grows as we scroll, since rendering out of range cells
  extends @_data with empty rows
• upgrade dependencies
• add context dev dependency