Minor Updates and Improvements

See the README: https://github.com/U-C-S/MiniDB/tree/v0.3.1#readme for usage and working

Feature Updates:

• Renamed delete cmd to drop and delete is now used to delete the data
• read cmd for reading only specified ids
• Add help command

Development Updates:

• Create constants.errors class
• Use XPath API
• Use abstract class instead of override
• Code commenting