Update MultipleValueOutputFormat to handle blank keys.

Updated the MultipleValueOutputFormat to handle blank keys. The issue relates to hadoop streaming and being unable to disable the insertion of a trailing tab character in the output. Since my process is writing out a text delimited flat file ready for DB ingestion the extra tab character ends up on an row by itself. This causes MultipleValueOutputFormat to choke thinking that it is a properly formed key/value pair. To handle this I edited MultipleValueOutputFormat to stick all blank keys into a hardcoded default "blank-key-records/" directory. My clojure skills are non-existent, but may want to have this configurable by a command line option.

https://groups.google.com/forum/?hl=hy#!topic/mrjob/CbzhPfZJzF4
http://stackoverflow.com/questions/18133290/hadoop-streaming-remove-trailing-tab-from-reducer-output