nsqd: protocol_v2 add index check for reader command slice

* add a length check to prevent 'index out of range' panic when nsqd
  receives a malformed command ("\n"), leaving len()-1 out of bounds.
Merge pull request #120 from michaelhood/solo_newline_crash_fix_120

nsqd crashes if a reader sends an "empty" command
docs: update changelog/contributors; bump Go lib to 0.3.0