overwrite PASS instead of appending

Merge pull request #3 from chrisamiller/master

replace PASS filter when needed, instead of appending

handle missing or zero depth field in tumor

v0.4.2

handle missing or zero depth field in tumor

a few bugfixes

v0.4.1

whitespace issues

handing missing DP/AD fields

v0.3

handling missing AD/DP values

added a sanity check

handles weird edge case of have DP=0 in both tumor and normal without crashing

initial release

v0.1

initial commit