Moved SortVCF to public
Generalized MergeVcfTest and added a unit test specific to SortVCF

MergeVcfTest had many of the input-validation tests I wanted to perform on SortVCF as well.

Did some minor cleanup to SortVCF as well.
SortVcf changes to address code review
Merge pull request #68 from broadinstitute/bt_sort_vcf_enhancement_PFB-981

Adding enhanced SortVcfs CLP