add wkhtmltopdf

Closes #6696.
Merge pull request #6698 from vitorgalvao/fix-6696

add wkhtmltopdf
Merge pull request #6680 from rolandwalker/doc_appcast

document `appcast` stanza
Merge pull request #6683 from rolandwalker/appcast_format

appcast :format for 4 Casks
comment out broken appcast in induction.rb
add appcast :sha256 in adapter
add appcast :sha256 in adium
add appcast :sha256 in amethyst
add appcast :sha256 in apikitchen
add appcast :sha256 in appcleaner
add appcast :sha256 in arranger
add appcast :sha256 in audiobookbinder
add appcast :sha256 in aurora
add appcast :sha256 in backuploupe
add appcast :sha256 in bbedit
add appcast :sha256 in bibdesk
add appcast :sha256 in bit-slicer
add appcast :sha256 in black-ink
add appcast :sha256 in blueharvest
add appcast :sha256 in bootxchanger