Rename limit to content_limit. Add limit.

`limit` is a reddit parameter of how many results are returned with every requests. PRAW also uses `limit` as a parameter for a lot of methods. So it gives confusing function calls like ´subreddit.get_hot(limit=None, params={'limit': 100})` and for people familiar with the reddit api. So I renamed `limit` to `content_limit` and added a limit parameter that works like the upstream `limit` parameter.