Added ExpenseClaim and Receipt models

Hello Wayne,

I implemented the following models: ExpenseClaims [1] and Receipts [2].
I was based on the current models. I might have missed something, please let me know.

I didn't add any tests yet, but I used this fork within my project and it pulled the data correctly.

[1] http://developer.xero.com/documentation/api/expense-claims/
[2] http://developer.xero.com/documentation/api/receipts/

Regards,
Exe.