fix invalid free

Thanks To DaveDavenport
Use sanitized variable instead of settings value
config for max history length

fix #177
add new history_length option to example dunstrc
Abort build if config.h needs to be updated

close #179

Thanks progandy
move id replace before stack_duplicates logic
remove whitespace
Merge pull request #183 from nopjmp/stack-replace-fix

Notifcations that replace another still 'stack'
Merge remote-tracking branch 'knobweb/master'

Also making up to spec with what I mostly want.
http://random.sarine.nl/rofication.html

Conflicts:
	dunst.c
	notification.c