fix: panic when listing session that have an invalid LastAttached time

[markfeinstein]

Change the behavior of StringToTime to return a zero value *time.Time rather than a nil to prevent a panic when sorting a slice of sessions.

fixes #34

Additional changes

• Update deps go get -u all
• Add testify package for unit testing
• Add functional and benchmark tests
• Refactor format function for performance improvements
• Extract session data processing code to improve testability and minor readability improvements

[joshmedeski]

Thanks for this work! I'll start using testify and will be adding more tests 👍