Move tags to their own directory

I've finally something to submit yey. So I've been working to implement and fix few things for a while, but because of how Chronic is currently designed that's not been easy at all. Basically this design just doesn't allow to support some things without making ugly hacks so I've rewritten quite a large part with a bit different approach. I didn't wanted to change that much and I didn't expected it will be such big amount of work. But basically this new design will allow very easily to add new date/time formats and fix all issues nicely. Now because changes are quite big and because I haven't fully finished everything (but enough to know this is right direction) I decided I'll split changes over several PRs (rather than one which I thought I'll have) thus it will be much easier to review.

About this PR, there's nothing special just move tag classes to `tags` directory for better organization.