fixes for deep coverage and better QC metrics.

v0.1.3

• if a sample had > 1 allele that was neither REF nor ALT at a given site, it was assigned
  an unknown genotype. This was too stringent for deep sequencing so it was changed to a
  proportion (> 0.04 [or 1 in 25 alleles]) #7
• for samples with sparse coverage, e.g. from targeted sequencing projects, mean depth is
  not very informative because it gets washed out by all the zero-depth sites. The new columns:
  gt_depth_mean, gt_depth_std, gt_depth_skew` report the values for the depth at genotyped
  sites--those meeting the depth requirement (default of 7).