Speed up ChunkyPNG with OilyPNG

From TFA:

  OilyPNG is a Ruby C extension to speed up the pure Ruby ChunkyPNG
  library. It is a standalone module, so it does not require LibPNG,
  ImageMagick or any other library. Currently it has an alternative
  implementation of decoding and encoding PNGs, making these operations
  much faster, especially for PNG images that apply filtering.

Performance comparison: https://gist.github.com/wvanbergen/611255

Since this is such a simple drop-in thing, I'm dropping it in.

Change-Id: Ida4ce27150e64a5e884122005cdf7eae10a4eccd