Fix case sensitivity issue in VariantHeaderBuilder that was preventing shared/individual field types from getting set

Quick fix needed by Joel
Merge pull request #298 from broadinstitute/dr_variant_header_builder_case_sensitivity_bug

Fix case sensitivity issue in VariantHeaderBuilder that was preventing shared/individual field types from getting set
Merge variant headers
- Merge bcf_hdr_t with merge_variant_headers()
- Merge VariantHeader with VariantHeaderBuilder::merge()

Add VariantHeader equality operators