tmux 2.0 support

[tony]

https://travis-ci.org/tony/tmuxp/jobs/60598709

======================================================================
ERROR: test_error_version_less_1_7 (tmuxp.testsuite.util.TmuxVersionTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/tony/tmuxp/tmuxp/testsuite/helpers.py", line 154, in setUp
    self.bootstrap()
  File "/home/travis/build/tony/tmuxp/tmuxp/testsuite/helpers.py", line 176, in bootstrap
    s.get('session_name') for s in t._sessions
  File "/home/travis/build/tony/tmuxp/tmuxp/server.py", line 148, in _sessions
    return self._list_sessions()
  File "/home/travis/build/tony/tmuxp/tmuxp/server.py", line 121, in _list_sessions
    raise exc.TmuxpException(proc.stderr)
TmuxpException: [u'error connecting to /tmp/tmux-1000/tmuxp_test (No such file or directory)']

Lots of this.