Introduce and check for 'STABLE' build constants.

- We can now `#if (STABLE)` to see if we're on a stable branch.
- Stable information included in version info (eg `ckan version`)
- Travis enforced checks constants are defined if on or merging into a stable branch.
- Includes #316 ("any arch" fixes)
- For #302 (let's release)

Once this lands, we'll properly fork stable from dev, and I'll add a dev commit which removes the stable constants.