Raise KeyError in __getitem__ if key is missing

If the '__getitem__' method is called with a delimiter separated key
that's missing in the FlatDict, then KeyError is raised instead of
returning 'None'
Fix unittest

The 'test_contains_false' had the same code as that of
'test_contains_true'
Fix syntax of the example included in README
Remove unnecessary code
Merge pull request #1 from naiquevin/keyerror-fix

Raise KeyError in __getitem__ if key is missing and a few minor fixes