fix bug where a file containing duplicate blocks would fail to be read properly
style(merkle): move var dec closer to use
fix(merkle) use defer
Update merkledag.go
change FindLink to FindLinks

cc @maybebtc
some cleanup, use WaitGroup over channel ugliness
fix FindLinks comment
cleanup from CR
Merge pull request #447 from jbenet/dup-block-fix

GetDAG duplicate block fix.