Moar safety

  - Use 'safe' versions of 'int_of_string' and 'float_of_string'
  - Ensure 'REF' and 'ALT' values are valid DNA
Unified 'test_vcf', time for full-scale testing
Added a test for VCF sample from the spec.

  See http://www.1000genomes.org/wiki/Analysis/Variant%20Call%20Format/vcf-variant-call-format-version-41
Clarified unsupported features in 'Biocaml_vcf'