ExifTool Parser

Adds an external parser that calls ExifTool to extract extended metadata fields from images and other content types.

In the core project:
An ExifTool interface is added which contains Property objects that define the metadata fields available.
An additional Property constructor for internalTextBag type.

In the parsers project:
An ExiftoolMetadataExtractor is added which does the work of calling ExifTool on the command line and mapping the response to tika metadata fields. This extractor could be called instead of or in addition to the existing ImageMetadataExtractor and JempboxExtractor under TiffParser and/or JpegParser but those have not been changed at this time.
An ExiftoolParser is added which calls only the ExiftoolMetadataExtractor.
An ExiftoolTikaMapper is added which is responsible for mapping the ExifTool metadata fields to existing tika and Drew Noakes metadata fields if enabled.
An ElementRdfBagMetadataHandler is added for extracting multi-valued RDF Bag implementations in XML files.
An ExifToolParserTest is added which tests several expected XMP and IPTC metadata values in testJPEG_IPTC_EXT.jpg.