Flush document to disk if it does not exist

When a new file is created, the file server tries to read it from disk.
If it does not exist, it is a good time to flush the newly created empty
file.
[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