Add feature to extract keys from a pattern without needing to parse a string (with tests).
Add docstrings to new methods.
Rename test and reword docstring for clarity.

Also, adhere to PEP8 for spacing between module level functions.
Maintain line length limit.
Return set rather than list to better represent resulting data set.
Fix typo.
Add test for mutating side effects (currently failing).
Return copy of internal set to avoid external mutation.
Avoid abbreviations.
Mention new keys method in tutorial.
Merge pull request #13 from pull/13

Add feature to extract keys from a pattern without needing to parse
a string