reorder functions for code clarity

- Simple functions like status() and get() are moved to the top.
- Private functions used by post(), index() and others are grouped
  with those functions.
decompose elasticsearch.index() into smaller functions
docstring usage examples
only choose files with images to be signatures
Merge branch '137-browse-ui' of https://github.com/densho/ddr-cmdln into 137-browse-ui