Signtool

Fixed

- fixed `remote_signfile` exception logging.

Changed

- moved source repo to [mozilla-releng/signtool](https://github.com/mozilla-releng/signtool)
- changed required pefile version to `>=2016.3.28` because we need [this fix](https://github.com/erocarrera/pefile/commit/ac410dcf7fff6840a06bc50e374f4b4db33e0c0e) for py3.

Removed

- removed py26 support

Changed

- fixed packaging to remove tests

Added
- more unittests; 65% coverage

Changed

- switch to `requests` instead of `poster`+`urllib`
- refactor to be more clean architecture oriented
- switch to `pytest` from `nosetests`

Fixed

- py3 signtool working again

Added

- py3 support
- standalone signtool support

Changed

- Copied signtool source files from [build-tools](https://github.com/escapewindow/build-tools/commit/2c797a5623efc391cd09304d314571638b596e8c)