Mixed-case checksum address encoding

[dahlia]

The current Address object provides hexadecimal formatting, but it always returns only lowercase letters. Since we are following the same address scheme to Ethereum by design, we should follow EIP 55 as well.

This task consists of two subtasks:

• Making the formatting method to return a mixed-case EIP 55 string. → Implement the formatting method to return a mixed-case EIP 55 string #43
• Adding a new constructor to take a hexadecimal address string. If a string is mixed-case and the checksum is invalid it should throw an exception. → Add a constructor which takes a hexadecimal address string #53

[dahlia]

Closed by #43 and #53.