Don't show empty lists of matches

Fixes https://github.com/rking/ag.vim/issues/40
Merge pull request #42 from losingkeys/no-empty-lists

Don't show empty lists of matches
Whitespace/formatting
Add an option to suppress the mapping message

Fixes https://github.com/rking/ag.vim/issues/39
Merge pull request #41 from losingkeys/mapping-message-option

Mapping message option
Merge branch 'master' of https://github.com/rking/ag.vim

* 'master' of https://github.com/rking/ag.vim:
  Add an option to suppress the mapping message
  Whitespace/formatting
  Don't show empty lists of matches

Conflicts:
	autoload/ag.vim