Better support for hopper and hopperchain specs.

Added module-level functions to convert hopper/hopperchain specs
to and from strings.
Stopped shadowing "dir" in _find_jpgs_in_dir.
Some PEP8 corrections.
Minor bug fixes and PEP8 corrections.
Some PEP8 corrections.
Added lots of docstrings and did some light rearranging.
More documentation.
Changed set_hopper to accept a hopper_spec instead of its two pieces.
Fixed test for new set_hopper behavior.
Merge pull request #8 from wil-langford/hoppers_better_hopper_and_hopperchain_specs

Better support for hopper and hopperchain specs.