Viz minor bug fix and some docs fixes.

Minor bug that can be reproduced when running example plot_from_raw_to_epochs_to_evoked.py 

When clicking outside axis on the plot with all the epochs- 
```
  File "/usr/lib/pymodules/python2.7/matplotlib/backends/backend_qt4.py", line 169, in mousePressEvent
    FigureCanvasBase.button_press_event( self, x, y, button )
  File "/usr/lib/pymodules/python2.7/matplotlib/backend_bases.py", line 1632, in button_press_event
    self.callbacks.process(s, mouseevent)
  File "/usr/lib/pymodules/python2.7/matplotlib/cbook.py", line 262, in process
    proxy(*args, **kwargs)
  File "/usr/lib/pymodules/python2.7/matplotlib/cbook.py", line 192, in __call__
    return mtd(*args, **kwargs)
  File "/home/prav/mne/mne-python/mne/viz.py", line 3250, in _epochs_axes_onclick
    here = vars(ax)[p['axes_handler'][0]]
TypeError: vars() argument must have __dict__ attribute
```
And some other small doc fixes.