Added deriving clauses to FetchResult.
Updated comic scripts

The program wasn't able to read them due to the changes in the
"LinearComic" datatype.
Added Fetch.Tree.extractResults for getting all leaves of an MTree.
Added comments to Helper.Functor.
Added <$=< and operator precedences to Helper.Functor
Closes #38.

Also changed the error type of all FetchResults from NetworkError to
[NetworkError].