Easier specification of external file dependencies#83

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ramnathv wants to merge 7 commits intomasterfrom feature/file-dependency

+151-11

Commits on Jan 16, 2015

initial work on easier file dependencies.
ramnathv
committed

Commits on Feb 19, 2015

some minor changes (thanks @yihui )
ramnathv
committed

Commits on May 28, 2015

Enhance attachment() feature to work with dynamic data
jcheng5
committed