TIKA-879 Add a new parent mime type, for the text based message formats, of text/x-tika-text-based-message, which allows Thunderbird messages to be correctly detected as they now show up as being text based not binary based in the hierarchy

git-svn-id: https://svn.apache.org/repos/asf/tika/trunk@1647721 13f79535-47bb-0310-9956-ffa450edef68