0.2.0

Features in `img-dup v0.2.0`:

- An invalid image will no longer crash the thread it's processing on (Closes #13)
- Invalid images are reported in the results
- All IoErrors are now reported (Closes #12)
- There is now an option to output results as JSON, either to the outfile designated by `--outfile=` or to stdout. (Closes #3)
    - If printing JSON to stdout, all other messages are silenced so the output can be safely piped to a JSON parser.
- If `--outfile=` is present, `img-dup` will now test if the path is valid and writable *before* beginning processing. (Closes #7)