Replace SA_NOMASK with SA_DEFER

OS X and FreeBSD don’t have SA_NOMASK.

Linux sigaction(2) man page says: "SA_NOMASK is an obsolete,
nonstandard synonym for this flag [SA_DEFER]"
Add .gitignore
Add ATmega1284P to device list
Rename source directory
Update README
Merge pull request #2 from nils-eilers/master

Minor changes to run on OS X as well