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.
Having a separate snippet for the else seems a bit too much, what if instead we just do

    # -*- mode: snippet -*-
    # name: if
    # key: if
    # --
    if $1 {
      $2
    }${3:else {
      $4
    }}
    $0

In this way the else becomes optional and you don't need different snippets..
Or is it a common operation to add an "else" after an already existing if?