Feature: Order Callbacks & Cancel Order

[f3rno]

This PR adds a cancel_order method to WSv2, and support for callbacks in both cancel_order and submit_order that resolve with received notifications. Also adds request_calc, and comments/doc headers to WSv2.

Example:

client = Bitfinex::WSv2.new({
  # ...
  :transform => true
})

client.on(:open) { client.auth! }
client.on(:auth) do
  p 'succesfully authenticated'

  o = Bitfinex::Models::Order.new({
    :type => 'EXCHANGE LIMIT',
    :price => 3.0152235,
    :amount => 2.0235235263262,
    :symbol => 'tEOSUSD'
  })

  client.submit_order(o) do |order_packet|
    p "recv order confirmation packet with ID #{order_packet.id}"

    client.update_order({
      :id => order_packet.id,
      :price => '3.0'
    }) do |update_packet|
      p "updated order #{update_packet.id} with price #{update_packet.price}"

      client.cancel_order(order_packet) do |canceled_order|
        p "canceled order with ID #{canceled_order[0]}"
      end
    end
  end
end