Added changes to call callBackEndSync only when all SQL transaction has been commited
Added progress callback support (upload/download)
Added 'Accept-Encoding' header for supporting gzipped responses from the server (a must with big json)
Merge pull request #29 from jacargentina/master

New features thanks to jacargentina: Added progress callback support (upload/download)
Added 'Accept-Encoding' header for supporting gzipped responses
Added changes to call callBackEndSync only when all SQL transaction has been commited