Signingscript

Fixed
- compressed zipfiles

Added
- windows zipfile signing support.
- `SigningScriptError`
- py36 test support

Changed
- Moved the `aiohttp.ClientSession` creation into `async_main`
- No longer close the event loop at the end of `main`
- `sign_file` no longer takes a `to` kwarg; we always overwrite the original file, due to zipfile signing logic.

Fixed
- `pytest-asyncio` 0.6.0 compatibility

Added
- `example_server_config.json`
- 100% test coverage, with full docstrings and `flake8_docstrings`
- moved `SigningServer` named tuple out of function, for easier importing and reuse

Changed
- no longer accept `dmgv2` format
- explode and tar dmg files (support dmg signing in taskcluster)
- `get_default_config` now takes a `base_dir` kwarg
- moved `_execute_subprocess` to utils
- `sign_file` now returns the path to the target file
- `async_main` now copies the returned path to the `artifact_dir`

Fixed
- close the event loop on `main()` exception

Fixed
- Fix an execution error due to the addition of zipalign

Changed
- zipalign APKs in order to allow them to be published onto Google Play Store

Changed
- look at `work_dir` for the downloaded artifacts, to match the latest scriptworker changes