Implement Limit api parameter in Listing<T>

Implemented a new constructor in ListingEnumerator<T> and its returning
query in Listing<T> to allow the user to GetEnumerator(limit) where that
limit will be passed in the API query to reddit.  Implemented count
though it isn't truly necessary.
Formatting

Spaces after if statements.
Merge pull request #83 from furiousgeorge/master

Implement Limit api parameter in Listing<T>