give links default strength of 1
Merge branch 'master' into z-fixes
refactor cdf to pdf
change link pdf threshold line color and width

had to improve rendering algorithm to deal with granularity issue
when rendering visible part of pdf right at threshold.
Merge pull request #75 from jdhenke/pdf-styling

link strength pdf styling
Merge pull request #71 from jdhenke/z-fixes

fixes based on will zeng's comments
Merge remote-tracking branch 'upstream/master'