Ability to enable dryRun mode in runtime

In the meantime I noticed that you already solved the issue with `ScmRepository` created in the constructor, so I just added `dryRun` flag which is much cleaner than setting `release.dryRun` property on a project in runtime. That should resolve #34.

To test it automatically in reliable way I would probably have to create a functional test and I didn't know if you like the idea.

By the way I added missing constructor parameter in one test, but I left it in a separate commit just in case you prefer longer assertion syntax.
Added `public` to tell Groovy not to generate getter for those constants.