Umi-tools

Improves the group command:
- Adds the --subset option as per the dedup command (74)
- Corrects the flatfile output from the dedup command (72)

The code has been tweaked to improve run-time.
See 69 for a discussion about the changes implemented.

- Corrects the edit distance comparison used to generate the network for the directional method.
- This will only affect results generated using the directional method and `--edit-distance-threshold` >1

Previously, using the directional method with the option `--edit-distance-threshold` set to > 1 did not return the expected set of de-duplicated reads. If you have used the directional method with a threshold >1, we recommend updating UMI-tools and re-running _dedup_.

- Debugs python 3 compatibility issues
- Adds python 3 tests

Minor bump:
Resolves setuptools-based installation issue

Version bump to allow pypi update. No code changes