VariantHeaderBuilder: add_contig -> add_chromosome
Merge pull request #115 from broadinstitute/mc_contigs_highlander

VariantHeaderBuilder: add_contig -> add_chromosome
Sam: add tests to the public API

Added the following tests:

* unclipped start and stop
* copy constructor tests
* copy assignment test
* tags of types char, int and double
* char_tag when tag is not a char
* string_tag when tag is not a string
* is_missing support for tags

major strides towards addressing issue #4