fetch a group of posts that belong to a topic
fetch a group of posts that belong to a topic

Added new method for fetching a chunk of posts that belong to a topic.

Currently if you called `client.topic('topic_id')` you would only get
back ~20 posts.

Now you can call `client.topic_posts('topic_id', post_ids)` to get all
the posts you specify in the `post_ids` array.

techAPJ/discourse_api,spatten/discourse_api,oblakeerickson/discourse_api,RyzacInc/discourse_api,melissawahnish/discourse_api,Odyl/discourse_api,discourse/discourse_api