Add consistent (by filename/path) sort on hashes

To avoid randomization in version control, it's best to have a
consistent order of results in the assethash JSON object between runs,
even if the source files don't change.

This commit adds a step to the file output process to re-order the JSON
object based on sorting the keys.

It would be great if you could review + merge the pull request, and publish a new version to npm! Please let me know if you have any questions or concerns on the code.

This fixes Issue https://github.com/ktmud/grunt-hashmap/issues/1.