v6

• Delete zero package, there's not a great need for it.
• Add a Byte type (used for single bytes, for example a char in a database).
• Removed sql package null types in favor of custom types and implemented their value and scan methods. This allows us to simplify the structs.
• Merged structs for all other types into a single struct to make the API a bit cleaner.
• Fixed bugs and heavily refactored every types UnmarshalJSON.
• Removed support to unmarshal null types using UnmarshalJSON, i.e. "{"Int": 8, "Valid": true}" will no longer unmarshal successfully. Only support single value JSON now, i.e. 8 opposed to a struct representation.