LUCENE-4684: Made DirectSpellChecker extendable.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1433431 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-4684: Moved change entry log.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1433444 13f79535-47bb-0310-9956-ffa450edef68