Merge pull request #36 from nickhammond/bills

Add in support for the Bill and related models
change invoice.id to invoice.id.value
Merge pull request #38 from theinventor/master

fix for bad uri - invoice.id.value instead of invoice.id
Merge pull request #41 from nickhammond/vendor

Added Vendor support to QBO
Version bump
Added more contributors
Update supported Rubies and base URL documentation
Merge pull request #42 from nickhammond/patch-1

Update supported Rubies and base URL documentation