Implement Grails 2.3.x binding support

* Changes to plugin to support 2.3.x binding support. This means I needed to bump the grails dependency to 2.3
* Updated to latest joda-time dependency (which meant implementing pull request https://github.com/gpc/grails-joda-time/pull/2)
* Changed some tests so they would pass in timezones outside of Europe/London (and I think they would still pass there)
 * Incidentally [GPJODATIME-21](http://jira.grails.org/browse/GPJODATIME-21) is now appears to be fixed in Grails 2.3.x
 * All tests but the one marked [GPJODATIME-28](http://jira.grails.org/browse/GPJODATIME-28) pass