Fix quotation characters

Use ASCII quotation marks rather than fancy Unicode quotation marks,
as VCF files need straight ASCII ones and the fancy ones disappear
during TeX processing anyway.

Also remove excess spaces from ##INFO=<..., Source="foo", Version="bar">
example.
Added DS tag to @PG as it seems to be in common use.
Tidy up SNP/insertion/deletion/mixed VCF examples

Fix mixed VCF record typo (REF allele should be GCG, as in the wiki
version).  Hat tip @holyspidoo, fixes #6.

Clarify haplotype tables: C is only the reference base in the first
few examples; align sequence letters via verbatim monospaced font.
Merge VCF example and quotation mark fixes