Create a Tika-based MIME type extractor

The Tika library does a pretty good job of determining the MIME type (see the existing code in the Tika text extractor). In fact, it seems to be quite a bit better (with less JARs) than Aperture.

So we should create a Tika-based MIME type detector, and MimeTypeDetectors should automatically load it just like the Aperture-based one.