fix: loaded/saved/created/deleted at timestamps and selectors

[jrista]

• Change all timestamps in state to unix times number values
• Update selectors to create Date objects from unix times in state
• Clarified return types on selectors where null was a possibility
• Extracted projection mappers to discrete functions for testability
• Updated reducer unit tests

Resolves #98

[jrista]

I've extracted all of the projection mapper functions from the buildSelectorMap function. This will make each of them unit testable on their own, and more easily testable. I'd like to get unit tests written for each before we merge.