Added MultipleValueJSONOutputFormat

[aghadiry]

I added MultipleValueJSONOutputFormat, because MultipleJSONOutputFormat would write the key to the file. MultipleValueOutputFormat would not work because the key was JSON and would get treated as a string, resulting in directories being quoted.

[jblomo]

Do you mind trying to write a test for this? The general idea is to setup some Text objects and run #generateFileNameForKeyValue and #getActualKey with the new class to make sure they return the correct Strings. lein test will run them.