Also add some versioning policy. Basically, the policy I've decided on:

1. majorver has abstract high-level semantics (e.g. major rewrite)
2. apiver controls the public API (=soname), updated any time the public
   API is extended or reduced
3. fixver is updated any time a fix is made without changing the API

Additionally:
- fixver resets on apiver changes
- apiver never resets