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
bit-wise ops seem specific to CRAM internals only. What would be the use case of having this in samtools.io? I suggest to keep it under cram package.