Fix for --dry-run option not working

Running pg-migrate with the dry run option i.e `pg-migrate [up | down] --dry-run` does not modify pg-migrates's behavior. It sets 'global.dryRun' but this is never used by any code.

Here is a fix complete with tests. As regards testing I used [Mocha](https://github.com/mochajs/mocha) as the test runner (installed as globally) and [Rewire](https://github.com/jhnns/rewire) and [Sinon](https://github.com/cjohansen/Sinon.JS) to isolate the test code.

I am happy to change my pull request if you want to use some other test framework.