Using a single-shot timer with the Wx backend raises an AttributeError

With the WxAgg backend, the following results in an error (it works properly on any other backend):

    import matplotlib
    matplotlib.use('WxAgg')
    import matplotlib.pyplot as plt

    def ping():
        print 'ping'

    fig, ax = plt.subplots()

    timer = fig.canvas.new_timer(interval=1, callbacks=[(ping, [], {})])
    timer.single_shot = True
    timer.start()

    plt.show()

This yields:

```
Traceback (most recent call last):
  File "wx_timer_issue.py", line 10, in <module>
    tim.single_shot = True
  File "/usr/local/lib64/python2.7/site-packages/matplotlib/backend_bases.py", line 1164, in _set_single_shot
    self._timer_set_single_shot()
  File "/usr/local/lib64/python2.7/site-packages/matplotlib/backends/backend_wx.py", line 206, in _timer_set_single_shot
    self._timer.start()
AttributeError: 'Timer' object has no attribute 'start'
```

This appears to be the result of a simple typo in wx_backend.py.