Make it easier to capture-pane

[tony]

Allow use of capture_pane as a property and automatically return stdout.

>>> import libtmux

>>> server = libtmux.Server()

>>> server.findWhere({'session_name': 'foo'})
Session($5 foo)

>>> session = server.findWhere({'session_name': 'foo'})

>>> session.attached_pane().cmd('capture-pane', '-p').stdout
[u' ______________________________________________________________', u'/ A light wife doth make a heavy husband.                      \\', u'\\                 -- Wm. Shakespeare, "The Merchant of Venice" /', u' --------------------------------------------------------------', u'        \\   ^__^', u'         \\  (oo)\\_______', u'            (__)\\       )\\/\\', u'                ||----w |', u'                ||     ||', u'~', u'\u276f']

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