Add getDetector() method to TikaConfig

As discussed on the mailing list, we should add a getDetector() method to TikaConfig. This would return a DefaultDetector that was created with the same classloader as the DefaultParser was

As part of this, we should update the Tika class to get the DefaultDetector from the TikaConfig, rather than creating internally. We should also switch the Tika class to not create its own AutoDetectParser, but instead use the DefaultParser from TikaConfig

Discussion is:
http://mail-archives.apache.org/mod_mbox/tika-dev/201110.mbox/%3Calpine.DEB.2.00.1110171330160.7762@urchin.earth.li%3E