Add Unlock Hash From String Utility Function

This merge request is to move the scanAddress function from ./node/api/scan.go to ScanAddress in types/encoding.go and to add the UnlockHashFromAddrStr function to types/encoding.go. This will allow easy access to these functions throughout the project and, most importantly, in types/constants.go. Having access to these functions in types/constants.go will allow the use of the unlock hashes in string form which is the form that most users will be familiar with. This in turn will help with ease of documenting code and writing tutorials.