adds initial support for the interfaces api
adds initial support for the interfaces api

This commit adds the initial support for the interafces api.  This api provides
configuratio access to working initially with Ethernet interfaces.  To work
with the interfaces api use the following:

require 'rbeapi/client'
node = Rbeapi::Client.connect_to('veos01')
interfaces = node.api('interfaces')
interfaces.getall

arista-eosplus/rbeapi,arista-eosplus/rbeapi