More sensible .cabal file so microhs can parse

[vmchale]

Right now the .cabal file has a list of extra-source-files that is separated by commas in one place but not another (this causes an error in microhs, viz.

mcabal: Building in /Users/vanessa/.mcabal/packages/dlist-1.0

mhs: uncaught exception: error:
  found:    't'
  expected: ','
  file: "dlist.cabal"
  line: 18
  at: "tests/ImportUnsafe.hs\nbuild-type:             Simple\ntested-with:            GHC==7.0.4\n            "

)