Release 0.8.0

Prepare for release 0.8.0

TODO
----------
- [x] Update empower to 0.8.0
- [x] Update power-assert-formatter to 0.8.0
- [x] Update other dependencies
- [x] Update implementation
- [x] Maint tests
- [x] Dedupe and rebuild web bundle
- [x] Say farewell to UMD
- [x] README
- [x] CHANGELOG