Add parallel processing to OCR text extraction of full documents

Leverage the GNU Parallel gem to OCR multiple pages in parallel.  If Parallel is installed, a full document extraction will generate an image for each page and then spawn a tesseract process for each available core.  If Parallel is not installed or a subset of pages are indicated, the old behavior will be used.  This speeds up OCR processing significantly on multi-core machines.

With a bit more work, this could be leveraged by the other OCR code paths.