Improvements to search-notes.py

 - Bug fix: Output encoding of listed keywords
 - Bug fix: Don't print empty line when there's no matches
   (related to issue #47 but not a complete solution ... yet)
 - Move index version to variable

See also issue #47 on GitHub:
  https://github.com/xolox/vim-notes/issues/47
Bug fix for communication between notes.vim and search-notes.py*

The problem was that there was no way to distinguish the two cases:

  1. The script successfully reported no matches.
  2. The script failed with an error (or couldn't even be executed).

Now the two cases are properly disambiguated.

See issue #47 on GitHub:
  https://github.com/xolox/vim-notes/issues/47