Handle custom yarn tar ball location; make yarn/dist/bin/yarn working again

[WesleyRen]

Summary fixed two thing:

#504: Let user specify a custom location of yarn dist tar ball. #647: copy installed /node/yarn/yarn- to /node/yarn/dist for execution path. Tests and Documentation Updated README

[pjjanak]

@eirslett - can this be looked at? It's a blocking issue for us (and probably others) - being able to specify the format for downloading yarn more explicitly would solve the problem.