ENH: iohub keyboard event comparison

iohub keyboard events now support direct comparison, using the event.key
field to determine equality.

So what use to need to be done as:

+         kb_evts = keyboard.getKey(['a','b','c'])
+         for event in kb_evts:
+             if event.key == 'b' or event.char == 'b':
+                 return True
+         return False
+

can now be done simply as:

+         if 'b' in keyboard.getKey(['a','b','c']):
+             return True
+         return False