http://stackoverflow.com/questions/13537126/term-frequency-in-lucene-4-0
http://blog.mikemccandless.com/2012/03/new-index-statistics-in-lucene-40.html
http://lucene.apache.org/core/4_0_0/core/org/apache/lucene/index/package-summary.html#termstats
나중에 별도로 포스팅 예정..
and mig
http://lucene.apache.org/core/4_0_0-BETA/MIGRATE.html