Made "itemremoved" event preventable

I searched 'infinite carousel'. Though there were some projects, your infinite-carousel is most simple and very effective!    
I added two changes for this pull request. One is about your comment, '//Todo make that configureable'. I suppose that it is better to calculate the distance from length of items than adding the distance to config.
Second is a way to prevent to remove an item. For our project, I would like to make items re-used and stop infinite on obvious start/end. I changed code to use fireAction instead of fireEvent for 'itemremoved'. In this case if 'itemremoved' is handled by onBefore and it's callback returns false, the item won't be removed.