Rename lib file to match the specified in gemspec

Hi there,

Thanks for the gem. I started taking a look at it today to add more models / services available on quickbooks v3 and  noticed that the file in lib is not named *quickbooks-ruby.rb*. So I wonder if that is intentional? One of the side effects here is that the gem is not required / loaded when I add it to my gemfile as I would expect. Instead I have to manually require it.

looking forward to work with this lib and quickbooks api :beers: