Simplified version number

From now on, AVIM versions take the form r.X.Y.Z, where X is a major version number, Y is a minor version number, and Z is a build number. The r stands for release; it’s part of the version string mainly to fool Mozilla Add-ons’ version comparison. The first such version string is r.5.3.0, chosen to look newer than 20080728.526b1.

Fixes #95.