Add limit param for listing leads

[AndrewNg]

Per https://developer.close.com/resources/leads/#list-or-search-for-leads, getting leads also takes in a limit parameter.

[taylorbrooks]

@AndrewNg This change is a breaking change because you're shifting/changing the positional arguments.

I'm not sure changes to the gem are required. You could do something like this in order to explicitly set a limit:

client = Closeio::Client.new(ENV['CLOSE_API_KEY'], false)
client.list_leads("admin@example.com", false, false, {:_limit => 10})

[AndrewNg]

ah ok that does work just fine, my apologies! appreciate your help :)