Merge pull request #2 from williambogaiv/master

Condense if-logic when checking whether top-level variable exists.
Create README.md
Remove explicit dependency on window
Merge pull request #3 from jrusbatch/window-dependency

Avoid directly depending on the global window variable
Use `this` context for stockRetriever passed in items
Remove unnecessary falsey check
Wrap long url
Avoid calling toUpperCase on null or undefined
Remove unreachable chaining code
Refactor to encapsulate configuration selectors.
New behavior only publicly returns necessary functions while wrapping configuration as a read-only function.
Chainable functionality is preserved.
Update tests to use new configuration method.