Trim line ends
Fix typo in factory

factory was not spelled right. Since it's a new property, I feel safe
renaming it without supporting the old one.
Added tests for storage

With initial test based on mock implementation
Make set check for falsy values

And save and empty string instead, because both session and local
storage are string based
Added david dm badge

For dev deps only
Local and session storage tests

This includes a refactor of the original mock test to be able to reuse
the same test logic across multiple storage engine.
More get/set tests
clear tests (not passing)