bugfix: pos is wrong when len(block) < 3
bugfix: dict can not been changed while iteritems()
clean workdir in child process

shutdown executor when reach max idle cycle
bugfix: empty line when reach end of file
reduce needed fds, connect when need
exit cleaner process
try reduce when got result
add options to keep_order (default is no)
hotfix
the worker process exit when executor had exited
refactor, using spawn
clean thread should not been daemon
start clean thread when init
fix hash collision of tuple
compress broadcast block by block

support object large then 2G
add taskMemory for groupByKey, reduceByKey, join
specify task memory for a stage

add taskMemory arguments for groupByKey(), reduceByKey(), join(),
groupWith() to specify the memory used by a task.
Merge branch 'master' of github-ent.intra.douban.com:davies/dpark
Merge branch 'master' into release