Re-factor the Python 2 Livestatus library to work under both 2/3:

* Replace some py2-only idioms and fix some errors like trying to access
  `.keys()` on a list.

* Let all literal strings be unicode on both py2/3
  (`from __future__ import unicode_literals`). This way, the modules
  code internally works with unicode stings (`unicode` on py2, `str` on
  py3) and converts to byte strings when writing to socket.

* API methods take either unicode strings or byte (`str` on py2, `bytes`
  on py3) as input.

Signed-off-by: Aksel Sjögren <asjogren@itrsgroup.com>