feat(job): store dividend history from NASDAQ

Parses HTML from a company's dividend history page.

Closes #13.
test(job): store dividend history from NASDAQ
feat(model): add store method to dividend

In an effort to make the code more DRY, a store method was added to the
Dividend model. This method checks if the dividend exits by symbol and
exdate. If the dividend exits it is updated otherwise a new dividend is
created.
fix(job): change dividend calendar storage logic

Closes #15.
Merge pull request #18 from davidcaseria/dividend-history

Dividend history job logic