Add discount_rate to line_items and include in calculations.

Fixes #56.

@waynerobinson  this isn't perfect yet, particularly it's untested, but I wanted to open it to start the discussion anyway.

It's not clear to me why we manually calculate the line item amount, when xero appears to return it from the API. [line_item.rb/33](https://github.com/latentflip/xeroizer/commit/784ff8fa0b8135d7a601dc3d3e978815c5f8ae8a#L0L33) Is there a reason why we recalculate it?