Faster index generation and customizable amount of replies in preview

I've modified a bit the buildPost function. It does not longer use latestRepliesInThreadByID (I don't see why use it if you're already using postsInThreadByID to get all the posts in order to count them).

I have also added a new constant (TINYIB_PREVIEWREPLIES) to configure the amount of posts to preview on the index files.