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
Variant: add Genotype getters

Based a Genotype class on VariantFieldValue, with a couple GT specific operators, including separating out the alleles.
Heavily based off htslib and comment stream in #77.
SamHeader: add n_sequences member function

* add n_sequences member function to allow iteration over the reference sequences
* make const member functions const (they weren't for some reason)
* inline simple member functions
Merge pull request #143 from broadinstitute/mc_sam_header_fixes

SamHeader: add n_sequences member function
Merge pull request #133 from broadinstitute/ks_genotype_field_value

Based a Genotype class on VariantFieldValue
Variant: read api for format field strings

There was a bug with the implementation that wasn't concordant with how
htslib parses the strings in the bcf1_t structure. It now works by
checking for bcf_string_vector_end values to separate the strings, and
skips blocks to find the values for every sample using the fixed length
blocks as defined by bcf_fmt_t->n.

Support for vector of strings per sample is not yet agreed upon in the
VCF/BCF spec, so comma separated strings will be treated as a single
string by htslib and thus by Gamgee.

* string type number of bytes per value is reported by fmt->n (weirdly)
* make format utility functions not inline (too big to inline)
* add tests for variable length strings across samples
* add functional tests for iterators of VariantField<VariantFieldValue<string>>

fixes #78