PHP-Xero updates

[ghost]

Hi,

Thanks for writing this class, it's been so helpful. I did encounter a few bugs and issues since using it, which I've fixed in my repo. These add proper support for PDF, get rid of a lot of php errors stemming from the class expecting xml where xml wasn't returned, fixing issues with authentication and various other gotchas I encountered.

It'd be awesome if some of these changes could flow upstream to your repo (since that's where everyone goes first when they start using the xero api).

Thanks!