[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
[TASK] update sjcl

- native support for k256
- improved entropy by taking advantage of platform crypto

- remove unnecessary sjcl overrides from sjcl-secp256k1.js
- updated ripple-lib curve instantiations to use k256
- add curve override so c256 points to k256

https://github.com/bitwiseshiftleft/sjcl/commit/16dde36fa2a58845f639429acd5bc4e4ebb5a0f1