Initial changes with Dave:q
Implemented the HeaderDiff's print method and test

Implemented the print method in such a way to be more descriptive in
explaining differences between vcf file headers.
Extracted all_columns method
Added to_hashref to Vcf Entry Object
Added EntryDiff.pm as well as associated tests
Added POD documentation for Differ, HeaderDiff
Changed HeaderDiff to include to_string subroutine
Ensured use of File::Vcf::Differ was correct
Changed Differ.t to allow for tests to run from any directory
include path in "can't open file" error message