Many makefile enhancements (support for direct invocation, instructions if invoked without target, release mechanism improved), package.json scripts now invoke the makefile.

- Using `npm test` and `npm run …` scripts defined in package.json now invokes the makefile and runs the test with the same shell that npm was invoked from.
- The makefile can now be invoked directly - supporting utilities from locally installed npm packages are automatically discovered.
- Invoking the makefile without a target errors out with a hint.
- Shell-specific test targets are now named 'test-<shell>'.
- Both 'test-<shell>' targets and the all-shells 'test' target now run all test suites by default.
- On `make TAG=<new-version> release` there must be no uncommitted changes. '<new-version>' can now also be one of the following increment specifiers: 'patch', 'minor', 'major'.
- It is ensure that <new-version>, if not an increment specifier, is a valid semver version number that is higher than the previous release's.
- The previous release tag is now located with a pattern so as to exclude tags that aren't version numbers.
- Switched from lightweight to annotated tags for releases.