Rename Serializers to Serialisers, because I'm just going to use British English spelling
Use the official XeroAPI schema repo as a submodule
Rename process_one / process_many to reflect what it actually does - e.g. serialise_one and deserialise_one
Update Payment serialiser: now that it's not being used where Xeroizer has monkey-patched BigDecimal.to_s, output the right kind of number strings
Move XSD related stuff for specs into spec/xsd_helper.rb

* Add Schema validator matcher
Add simple Invoice serialiser; update LineItem to work with it