[FEATURE] improve memo support

- add MemoFormat property for memo
- MemoFormat and MemoType must be valid ASCII
- Memo content is converted on the serialization level
- add parsed_* version of Memo content if the parser understand the format
- support `text` and `json` MemoFormat
[DOC] update generate wallet example

to take advantage of randomness collected from a rippled
Merge pull request #213 from geertweening/feature/memo_format_type

[FEATURE] improve memo support
[DOC] update release notes
[TASK] bump version to 0.9.4-rc1