json.RawMessage Metadata

• To make metadata fields more useful, the type was changed from map[string]interface{} to json.RawMessage (#19). Issue #16 (submitted by @yorhodes) has more context on why this is useful.
• This release also adds support for a deterministic hash function for comparing the equality of any two Rosetta types (#19). This is useful because the json.RawMessage type contained in many types causes reflect.DeepEqual to not work correctly as json.RawMessage is interpreted as a []byte instead of by whatever native go type it represents.