[fix #28] Accented characters are replaced by "?"

There was in fact two problems:
- Encoding was never specified when a file was read/written to FS
- Synchronization actor missed the proprer encoding handling in delta/raw
  command: mobwrite original specifications requires to be able to process and
  produce compatible input/output as the JS encodeURI function

Closes #28
[impl #23] Only Compile When Needed

Add method to retrieve last update date for a paper: it is the newest update
date for any of its document.