Updated helpers with new methods
 * Added method prepend to Helper::Path
 * Added method append to Helper::Path
 * Improved a bit code style in Helper::Program
 * The method canonicalize of Helper::Pathname now expand the path
   prior to canonilazation.
Added program model objects
 * Add program generic which will only update the PATH variable
   accordingly. Future specific program (like Java) will update more
   variables like JAVA_HOME et al.