Add more usefull snippets

I have added some snippets with the new rspec 3.0 syntax, and some snippets for factory girl's `let` and `subject`
I don't think there's a need to do this is `rspec-snippets-fg-syntax` has any value other than the one that means "auto-detect", right? Why not move the `with-temp-buffer` and `insert-buffer-contents` calls inside its branch?