VariantReader: fix deallocated memory access

converted string was going out of scope and getting destroyed, but its
internal buffer as returned by c_str() was continuing to be passed into
htslib's bcf_hdr_set_samples(), and eventually getting clobbered by new
memory allocations.

* add tests to for variants with missing genotypes
* rev-up htslib (taking the opportunity to get some bug fixes)

fixes #122
Merge pull request #129 from broadinstitute/mc_fix_n_samples

VariantReader: fix deallocated memory access
Variant: add id field getter

closes #107
Merge pull request #132 from broadinstitute/mc_variant_id_field

Variant: add id field getter
is_missing: add string missing value overload

fixes #134
Merge pull request #137 from broadinstitute/mc_missing_string_value

is_missing: add string missing value overload
Jamroot: make release the default build

* `b2` will now build the release variant, unless you specify `b2 debug` in the command line.
* `b2 test` will still build the debug variant by default and there is no way to run it in release mode (enforced)

fixes #128
Merge pull request #139 from broadinstitute/mc_build_release_by_default

Jamroot: make release the default build
Travis: general updates to the Travis scripts

* exclude pull-requests from updating dox fixes #140
* eliminate `sh` from the callout to scripts so env variables are passed in
* used env variables from travis scripts on sub-scripts instead of passing parameters around
* eliminate unnecessary if's from travis.yml and move them to sub-scripts
* use $CC instead of $TOOLSET as they were exactly the same and $CC is already set up by the environment
Merge pull request #141 from broadinstitute/mc_website_dox_update

Travis: general updates to the Travis scripts
Normalize all include guards

all include guards are now of the format :

`gamgee__file_name__guard`

fixes #130
Merge pull request #138 from broadinstitute/mc_include_guards

Normalize all include guards
VariantFieldValue: fixing EOF include guard comment
Merge pull request #142 from broadinstitute/mc_include_guards

VariantFieldValue: fixing EOF include guard comment
Typos in comments
ReferenceBlockSplittingVariantIterator
- uses htslib hack to modify Variant (TODO: use VariantBuilder)