Implement Employee BREAD API endpoints

[thejinbok]

The employee entity must have the following columns:

• id: Auto-incremented integer
• first_name: String, required
• last_name: String, required
• email: String, required, must be unique
• phone: String, optional
• department_id: Foreign key, related to the department entity

Remember that due to the nature of the relationship between the entities, the deletion feature must be soft, not hard.

[thejinbok]

I referred to the wrong issue on the commit above. It is related to #6.