Added leaf function to Data.Tree.Monadic.
Added System.Directory.Generic.renameFile.
Added Fetch.Types.Metadata

The file contains the types into which the metadata JSON gets decoded
(decoding function missing).
Wrote first tree transformation. See issue #54.
Made Fetch.Transform compile.
Added helper functions to Fetch.Types.Metadata.
Changed |>.

It was defined as (|>) = flip (.)
This was replaced with the correct definition (as used in F#): (|>) =
flip ($).
Schemata in #54 are finished (almost).

The function to read in the metadata file is not present.
Added to/fromJSON instances to MetaNode.
Closes #57.
Closes #58.
Closes #53.

Finished the functions structureByKey and readMetadata in
Fetch.Transform. Removed the stub of structureByTree.