Add an action to download a specific output of a build as a .nar.bz2

E.g. http://hydra/build/3515983/output/out downloads the output named "out" as a bzip2-compressed NAR.

Created by  Eelco Dolstra  on August 21, 2013
45A23NZPOQOJD4R45FPVPJR3642OCVIJHBA4RBQ3IB7JAQ2FGI4QC
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