More snippets for go-mode. Removed some verbosity in text of old snippets.

Just added some useful snippets for Go. Also changed argument names in old snippets for clarity.
I'm not too sure about this one because:
1. it only saves one key
2. this job is done better by things like smartparens (or auto-pair or whatever): https://github.com/Fuco1/smartparens
    Smartparens is in Elpa and it's quite awesome actually..