Change license from MPL to Apache 2.0.

This makes more sense than the MPL (which was added late) because many files
already have an Apache notice at the start. In addition to me, both Lloyd
Hilaiel and Philippe M. Chiasson are okay with the license change.
Add copyright notices to files that don't have them.
Update copyright notice years.
Merge branch 'upstream'
Fix typo in package description
Update debian/copyright on upstream relicensing
New upstream release 0.8.1