Pushover-complete

- Add [image attachment support](https://pushover.net/apiattachments>) (Pulls [5](https://github.com/scolby33/pushover_complete/pull/5>) and [9](https://github.com/scolby33/pushover_complete/pull/9>))
- Officially add support for Python 3.6
- Officially deprecate support for Python 3.5. It will be removed in the next major version release.
- Change default tox environment for Python 3 to py36
- Refactored `.travis.yml` to be more concise and use the new `py` [environment specification](https://tox.readthedocs.io/en/3.0.0/example/basic.htmla-simple-tox-ini-default-environments) (Pull [8](https://github.com/scolby33/pushover_complete/pull/8))
- Some refactoring in the main API (more list comprehensions yay!) (Pull [6](https://github.com/scolby33/pushover_complete/pull/6))
- Several small documentation changes/refinements

- "Add" Python 3.6 support. It's not in Travis as an allowed failure and didn't require any code changes to pass!
- Fix a major bug with the receipt cancel API. I was using a GET request instead of a POST
- Stop using the releases Sphinx plugin for the changelog. Its philosophy didn't match well with mine
- Update release procedure based on no longer using releases
- Some minor documentation fixes

- Officially add Python 2.7 support and add testing for it to tox and Travis
- Numerous updates to documentation and README, etc. to make them prettier and more useful

- Implementation of methods for the Pushover messages, sounds, users, receipt, subscriptions, groups, and licenses APIs
- Documentation and build process