add new recipes

- auto-complete-c-headers - ac source for completing the header file names
- ede-compdb - helper library to improve handling of C/C++ in CEDET
StackOverflow Search (SOS) mode for Emacs
google-calendar recipe
Merge pull request #1600 from dholm/recipe/google-calendar

google-calendar recipe
Merge pull request #1599 from pierre-lecocq/stack_overflow_search

StackOverflow Search (SOS) mode for Emacs
emacs-xkcd recipe
Merge pull request #1601 from ngnmnfn/master

add recipe for emacs-xkcd