Upload progress now works also for second file

- Sent bytes must be reseted before new upload, otherwise "onprogress" event "loaded" value provides sum of uploaded bytes (not just for the current upload, but all of them)
Merge branch 'wp8-loaded' of https://github.com/mhujer/cordova-plugin-file-transfer
Merge branch 'master' of https://git-wip-us.apache.org/repos/asf/cordova-plugin-file-transfer