Add Post#is_first_post?

We should be able to ask a post if it's the first in a topic
Add Topic#max_post_number to find max post number
Add Topic#url for determining url for a Topic
Add test to verify posts are moved in transaction
Extract PostMove from Topic into its own class
Merge branch 'chrishunt/extract-post-move'

* chrishunt/extract-post-move:
  Extract PostMove from Topic into its own class
  Add test to verify posts are moved in transaction
  Add Topic#url for determining url for a Topic
  Add Topic#max_post_number to find max post number
  Add Post#is_first_post?