added some methods

-- now you cannot remove a thread from the default ThreadGroup
-- you cannot pass a file uri or a script element object as an argument to the constructor of Thread
-- fixed a problem with the private properties
-- added the method "valueOf" to Thread, which returns the passed callback function
-- added the method "each" to ThreadGroup so you can iterate over all threads in a group
-- added the method "filter" to filter threads of a group with a callback function or if their not running anymore, and returns them