Change fs system to reify fs plugins instead of default to file fs

This resolves the bug https://github.com/Factual/drake/issues/111 in that it makes the sample Filesystem work with only a trivial modification (see my other pull request against drake-echostep). 

I taught myself clojure to make this diff so I wouldn't blindly merge. Note that there are some specification related concerns here (should FS's have to deal with leading slashes and why is it "hdfs://" when it is "mock:/") that this diff does not address.