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)
CB-6491 add CONTRIBUTING.md
CB-6378 Use connection.disconnect() instead of stream.close() for thread-safety
CB-3440 [BlackBerry10] Update implementation to use modules from file plugin
CB-6706: Relax dependency on file plugin
Merge branch 'wp8-loaded' of https://github.com/mhujer/cordova-plugin-file-transfer