Follow standard crypto interface in the crypto package

[shuse2]

Description

Currently, crypto package GetKeys do not follow standard crypto library interface. For example, PrivateKey is []byte but it implements Public and Equal. we should keep the standard interface as much as possible

• crypto should have PublicKey PrivateKey type instead of []byte
• Refactor GetKeys to accept []byte instead of string
• Add Verify and Sign
• Refactor util functions (Hash, hexToString, hexToBytes, GetAddress etc) to be in different file (utils.go)

Acceptance Criteria

• Update comments and examples
• All function should be covered by unit test

[shuse2]

Related discussion: #73 (comment)