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
CramRecord does not extend SAMRecord unlike BAMRecord, so in this sense they are very different. I have plans for the CRAM counterpart of SAMRecord, so I'm changing this one to CramCompressionRecord.