Make __getitem__ raise a KeyError for missing keys

[dnoetzel]

In redis-py, attempting to use getitem with a non-existent key
causes a KeyError to be raised, while fakeredis returns None. This
change makes it so that getitem in fakeredis works the same way as
in redis-py.

Fixes #191

[coveralls]

Coverage increased (+0.003%) to 98.463% when pulling 56b1367 on dnoetzel:make-getitem-raise-keyerror into 8216b16 on jamesls:master.

[coveralls]

Coverage increased (+0.003%) to 98.463% when pulling 56b1367 on dnoetzel:make-getitem-raise-keyerror into 8216b16 on jamesls:master.

[bmerry]

Thanks, looks good!