/clock in

[martyychang]

This pr contains basic functionality for /clock in, which creates a Time Entry record for a given Slack user based on the time the slash command was received. If the user tries to clock in a second time before clocking out, an error message is sent back to the user instead.

This pr also includes code that lays the foundation for future commands, notable with the following components.

• Slashclock.Command is the interface all commands should implement
• SlashclockUtil.getKnownCommands() contains the registry of recognized or known commands which slashClock is capable of processing

[martyychang]

Checked code coverage, which is at 84%