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
Merge pull request #293 from broadinstitute/jt_variant_header_merge

Merge Variant Headers
Travis: run tests in release mode in master only

create a new *production* mode to eliminate the -finline-functions that
is no longer supported by clang 3.5

also create a global fixture for initialization of global variables in
the variant reader test.

closes #290
Merge pull request #291 from broadinstitute/mc_relase_mode_testing

Travis: run tests in release mode in master only
Fix handling of vector end values by Genotype::operator[]

Vector end values were not passing through to the caller
when accessing alleles via operator[] in the Genotype class.
Merge pull request #302 from broadinstitute/dr_fix_genotype_handling_of_vector_end

Fix handling of vector end values by Genotype::operator[]
VariantBuilder
VariantBuilder code review round 2 update

-Addressed round 1 code review comments

-Enable move semantics when setting individual fields
 (allows for zero-copy bulk setting of a field for
 all samples at once)

-As a side effect of the above, setting individual fields
 by sample instead of in bulk is now supported. Uses
 short-value optimization to keep the per-sample values
 as contiguous as possible, but this option is still less
 efficient than setting in bulk.

-Significant refactoring to support the new move semantics
 and per-sample field setting:

    -split VariantBuilderDataField into VariantBuilderSharedField
     and VariantBuilderIndividualField

    -individual fields are now encoded at build time instead of
     at field set time; shared fields are still encoded upfront

    -to allow setting individual fields by nested vectors and
     by sample to compete with setting it by flattened vector,
     had to write a lot of low-level encoding routines to replace
     the former strategy of converting to flattened vector then
     handing off to htslib

Remaining TODOs:

-Fix issues with float missing/end of vector values (these are
 proving very problematic)

-Build-by-merge support (Joel indicated that this would be helpful
 for his use case, after all)

-More tests (now that Coveralls is working again, I can see the
 coverage gaps clearly)

-Doxygen comments / documentation
Fix bug with float missing / end of vector values

Apparently we can't use the htslib bcf_float_missing and bcf_float_vector_end
values directly -- need to copy them into local variables before use.
Fix bug in validation of vector lengths when setting individual fields

Previous validation routine would incorrectly allow nested vectors
whose first dimension was greater than but divisible by the number of
samples. Now validation of vector<vector> is separated from validation
of flattened vectors.
Improve test coverage, and fix issues found by new tests