Allow listing port-ins, and getting the TNs for a port-in

[singpolyma]

No description provided.

[ckoegel]

@singpolyma Looks great to me, I just added a test for the new method. I'll have someone else review as well but this should be good to go in our next release, thanks!

[ckoegel]

@singpolyma We would like to add some documentation for this new feature in the form of a code snippet in the README. I've created the snippet below to show an example of creating an array of all portins for an account. Was this usage of .next the way you intended, or is there another way of using the list method I'm unaware of?

client = BandwidthIris::Client.new('account_id', 'username', 'password')

@portins = Array.new
page = BandwidthIris::PortIn.list(client, {'page': 1, 'size': 5})

def get_portins(page)
    page.each do |portin|
        @portins.push(portin)
    end
    unless page.next.nil?
        get_portins(page.next)
    end
end

get_portins(page)

[singpolyma]

Yes, that looks very similar to how I am using it in my project