Org-sync json utilities

Hi daimrod,

since your org-sync repo seems to be the most active, please consider to merge these changes:

Added new utility functions for json parsing and respect the data's encoding. I had problems using org-sync-rmine with umlaut-containing bugs, so these changes became necessary.  To avoid code duplication, the new functions were put into their own package.  The code of the other backends was updated accordingly.