Merge code from Min zhou, includes #73 #82 #80 #79

[coderplay]

• Remove the package of dag

• 73 Add Mapper, Reducer and Partitioner

• Redesign EventProducer and EventEmitter
• Move the register method from AbstractWatchService to WatchService
• Remove the temporary Mapper and Reducer introduced by Jiannan
• WatchService should start watch quickly

• 82 Add child executor, which can execute map or reduce childs

• Let user can defined the mapper class and reducer class of a dragon job
• Delete SequenceFileEventProduer, because the inferfaces of EventProducer has changed