Refactoring: ApkLabelSet and ApkSentenceSet have same structure.

Due to almost same structure ont only between ApkLabelSet and
ApkSentenceSet but also ApkLabel and ApkSentence, I made new Class
ApkContent and ApkContentSet for dealing them.

Also, ApkContentEvaluator was modified to deal label and sentence
both.

Usage:
$ ruby ace_driver.rb demo/data/sentences/sentences.txt demo sentence apk
eval_apk.txt

Fixed #3, #5, #6