[FEATURE] allow per transaction fees to be set
Merge pull request #222 from boxbag/transaction-set-fee

[FEATURE] allow per transaction fees to be set
[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

[FIX] double serialization overriding Memo contents

The copy made in from_json wasn't a deep copy
Merge pull request #223 from geertweening/feature/memo_format_type

[FEATURE] improve memo support