Update README.textile to match the actual API
Update README.textile to match the actual API

There is a single `get_invoice` method that either accepts an invoice id
or number and not individual methods as described in the current README.

Fixes #27
Flatbook/xero_gateway,sharesight/xero_gateway,xero-gateway/xero_gateway,amxsv/xero_gateway,armstrjare/xero_gateway