- Fixed some minor error in the category api (the json structure returned
  has still the "todo" field instead of a "cat" field
- Added a full test script for the categories
- Added the ability to fork n thread, using cluster (experimental module).
  The number of thread is give in the config file, if it is missing, it will
  fork a number of thread equal to the number of core/cpus
- Splitted the user_api as stand-alone
- Small reorganization of the source tree (the APIs implementation are now
  in a subdirectory)