Improved the ability to get submissions from a subreddit (or set of subreddits)

You can now do something like:

```
Subreddit sub = new Subreddit ("funny", user);
LinkedList<Submission> items = Submissions.getSubmissions(sub, SortOption.TOP, TimeOption.WEEK, 10, user);
```

To get the top 10 posts this week in the /r/funny subreddit.

You can also do:

```
LinkedList<Submission> items = Submissions.getSubmissions("funny+wtf", SortOption.TOP, TimeOption.WEEK, 10, user);
```

To use a collection of subreddits.

I have updated the other examples to make use of this.