Merge pull request #174 from arthurchui/sandbox-mode

Support sandbox mode
0.1.6 release
Throw ServiceUnavailable for HTTP 504s as well as HTTP 503s
Catching invalid XML responses and returning a helpful error
Merge pull request #176 from harvesthq/invalid_xml_responses

Invalid xml responses
Added definition of entity type
Added Quickbooks::Forbidden for HTTP 403
Merge branch 'error-401-403'

* error-401-403:
  Added Quickbooks::Forbidden for HTTP 403