LUCENE-4684: Made DirectSpellChecker extendable.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1433438 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-4684: Moved change entry log.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1433443 13f79535-47bb-0310-9956-ffa450edef68