Accept subreddit input in various forms

I was using RedReader today and went to select a custom subreddit. I input "r/denver" and got an error stating that this was an invalid subreddit. I thought it would be cool if the input were flexible enough to understand "/r/denver", "r/denver" or "denver", so I made a quick change to do this.

I thought a static util method on <code>RedditSubreddit</code> which normalized the names seemed like a good solution. I was then able to use the normalized name where the code was concatenating "/r/" with the last part of the subreddit name in a call to that class' constructor.

I tested this out and it looks to be working as expected.