Submissions implemented

This will be my first pull request for jReddit.
I'm currently doing some research into Reddit and required a Java wrapper for the API.
I noticed you do not have sufficient support for Submission retrieval yet, so I implemented it.

Some things I want to discuss:
- There is a 30 requests/min limit in the API Access Rules, how does jReddit maintain that? Also where does it fetch the Response headers which contain how many requests are left?
- During the request, some things might shift, like popular constantly changing, only the Sort.NEW is consistent over time. Is this a problem?
- I added some minor JavaDoc issues and a general Exception throw

In summary:
Submissions subreddit posts retrieval implemented
Small change in pom file
Added more comments in Thing
Added JavaDoc to User
Removed unnecessary overrides in UtilResponse
Added new Submissions example
Is a time limitation required? Or shouldn't it be implemented in the class that creates the request to make sure the request limit is not breached?