Merge pull request apache#366 from colorant/yarn-dev

More yarn code refactor

Try to retrive common code in yarn alpha/stable for  client and workerRunnable to reduce duplicated codes. By put them into a trait in common dir and extends with them.

Same works could be done for the remaining files in alpha/stable , while the remainning files have much more overlapping codes with different API call here and there within functions, and will need much more close review , aslo it might divide functions into too small trifle ones, thus might not deserve to be done in this way.

So just make it run for these two files firstly.

Author: tgravescs