v2.0.0

What's Changed

• Split and organize the codes by @osamingo in #11
• Split error definition by @osamingo in #12
• Add NullString and use json.Number by @osamingo in #13
• Implement the method for Corporation API to kenall.Client by @osamingo in #14
• Bump up v2.0.0 by @osamingo in #15
• Edit testcases by @osamingo in #16

Full Changelog: v1.2.1...v2.0.0