Add GSP tags (link and createLink) that create short links

Here's what I did:

- Upgraded project to Grails 2.4.4
- Added ShortenerTagLib with link and createLink tags
- Added documentation for the new tags
- Added sensible default for characters and minLength if not specified in Config.groovy
- The sequence generator bean is compatible with the sequence-generator plugin
- First I introduced a breaking change but then I found a way to not introduce a breaking change

I tried to create integration tests for the tag library but did not succeed. Seems Grails 2.4 makes this very hard.
The createLink tag has been in production for over a week now so I'm confident it works.

Sorry for many changes in one pull request. I can try to split them if you want.