Make Keys, Values, and ItemsView subscriptable

[percurnicus]

Fixes #24

KeysView, ValuesView, and ValuesView now have a __getitem__ method so they can be subscriptable in a similar way that keys(), values(), and items() are subscriptable for normal dictionaries. I also adjusted their __repr__ methods to show the correct information in tandem with their dictionary method counterpart.

Example:

In [1]: import pvl

In [2]: module = pvl.PVLModule([
   ...:         ('a', 1),
   ...:         ('b', 2),
   ...:         ('a', 3),
   ...:     ])

In [3]: module.items()[0]
Out[3]: ('a', 1)

In [4]: module = pvl.load('tests/data/pattern.cub')

In [5]: module.keys()
Out[5]: KeysView(['IsisCube', 'Label'])

In [6]: module.values()[1]
Out[6]: 
PVLObject([
  ('Bytes', 65536)
])

[percurnicus]

All tests pass is ready for review @godber @wtolson

[godber]

@pbvarga1 this looks good. I think there's one suggestion that I would have for extending the tests. What if you had a bug in your implementation that always returned the first item instead of returning the correct item. Your existing tests would not catch that bug since you only ever test the first value. I will accept the PR without that change, however.

Also, we should do some talking with former stake holders about what to do with this project. I don't want you guys to have to rely on us who are increasingly less involved.

[godber]

Tests can be improved after the fact if desired. Accepting.