getItem returns null for unknown key

[psalaets]

This fixes #55

Changes

• getItem now returns null for unknown key
• Changed tests for removeItem and pull that were doing expect(...).toBeUndefined()

Note

When given an array of keys like ['known1', 'unknown', 'known2'], getItem returns [value1, undefined, value2]. Maybe that 2nd value should be null for consistency?

I left it as is since getItem(array) doesn't have an analog in localforage. Being different from localforage was the source of confusion in #55.

[ocombe]

Thanks!!