Attachments PUT

This is only support for uploading attachments to Xero objects using PUT (no listing or GET currently). As the Attachments API uses a different form of PUT request (body is entirely made up of file data), I have added a new separate method called put_attachment which seemed to make logical sense with the current layout of the Manager class. 

No new test coverage, but passes all current tests. Submitting in case useful for others and to start the ball on full implementation of Attachments API.

Best,
Bertil