The `STABLE` token can now be tested by #ifdef for stable relases

This allows us to do condiitonal compilation of code, so unstable
features can be removed from the compile.

- Aids in #302.
- Includes tests that fire when on stable branches.
- Stable/Unstable is now included in version information.
  (Eg: GUI title bar, or `ckan version`)
Stable constants check are now travis pull-request aware.

As long as it looks like we might merge into a stable branch, we
do our stability checks.
Merge pull request #317 from pjf/build_conditionals

Introduce and check for 'STABLE' build constants.