Remove the Body classes

There was no real need for separate classes here. Applying *Collapse
Hierarchy* refactoring from Fowler's book. Maintaining internal htslib
pointers to their respective headers inside the Variant and Sam classes
for performance reasons.
Merge pull request #47 from MauricioCarneiro/develop

Remove the Body classes
Variant: genotype_quals() implementation
Sam: add more docs to public API
hts_memory: add more docs to the public API
Variant: add more docs to the public API
VariantReader and SamReader: add filename to the example code