Sort files by EXIF date when possible

[rprieto]

Not sure what the best way is to implement this.

• how expensive is it to read EXIF data?
• can we afford to read it on every build, to sort the web pages?
• should we maintain an index of all files & dates, and only read files that are newer than the index?

metadata.json

[
  { file: 'media/2014/paris001.jpg', date: 135873927 }
  { file: 'media/2014/paris002.jpg', date: 135874139 }
]