replaced external xml2js lib with xml2json library

xml2json library is found in webos frameworks and does not need to be
suplied as node_module. Makes deployment much easier.
fixes to sync

- error handling did not work.
- removed throwing of exceptions, can cause endless loop of service
restarts. Better just return an empty array and remember error state for
next sync.
- fixed etag reading from return header.
added code to save etags after server update

now items don't get redownloaded if we just uploaded them.

Open-WebOS-DCO-1.0-Signed-Off-By: Achim Königs
<garfonso@tratschtante.de>