Cram code base

Just rebased the branch from master. Briefly:
- adds new htsjdk.samtools.cram package that contains CRAM backend like codecs, structures and thins
- adds CRAM type in SamReader
- CRAMFileReader and writer implementations
- changed visibility of TagValueAndUnsignedArrayFlag, used in the cram package
Do we want this public, perhaps maybe a getter, since this could be very useful in ```SAMIteratorForCRAM```?