Making tickets' custom fields available in the sprints' ticket lists.

This is a pull request for the topic that I originally submitted as issue #9.

Custom fields do not show by default, and can be activated in the preferences pane for each user. This is something that may be worth to rethink in the future, as it may be useful to decide which fields should be shown by default when installing/configuring the plugin.

I tried making the code as DRY as possible, and took the chance to refactor a couple of details. I see more that can be improved, but I'd leave that for a future refactoring.