Variant: rename format and info fields

 * eliminate is_hom_ref, is_het and is_hom_var from Variant. These should only be used inside the individual's Genotype
 * rename format_field to individual_field (also get rid of the generic_ prefix)
 * rename info_field to shared_field
 * rename builder and header member functions accordingly

fixes #123
Merge pull request #150 from broadinstitute/mc_rename_info_and_format

Variant: rename format and info fields