Add a default .gitignore for Rails

This seems to be implemented:

$ ruby railties/bin/rails zomg --dev
# snip
$ ls zomg/.gitignore 
zomg/.gitignore

I think the default .gitignore file rails produces uses a glob syntax that Git doesn't recognize. The template file includes this line tmp/**/*, but Git doesn't use the double-star pattern the way Ruby does. To ignore a tmp directory (and anything in it, including further subdirectories), shouldn't it be tmp/? (Or maybe /tmp/ if the idea is to ignore only the tmp directory at the top level.)