Setup release process

[kostko]

We should set up a release process, similar to the oasis-core and oasis-core-ledger repositories.

[kostko]

Preliminary automation/release process in #44, but it needs some improvements and tooling.

[pro-wh]

npm version minor --sign-git-tag seems to get us most of what we wanted in terms of tooling. it updates packge.json and commits the change and creates a signed tag. but would we prefer to sign the merge commit when we merge into master?