Add support for Quickeebooks::Online::Model::JournalEntry

Added support for journal entries within QBO. I also had to make a few tweaks to the pluralization of the model names, it was just appending an 's' to the model name.

I ended up with this:

<code>self::REST_RESOURCE.underscore.dasherize.pluralize</code>

What I'd really like is a <code>hyphenate</code> method but that doesn't exist. Can you think of a more efficient way to do this? The main use cases are converting JournalEntry -> journal-entries and SalesReceipt -> sales-receipts. So we definitely need <code>pluralize</code> on there to work with models ending in 'y'.