Implement Buy Process

- Api: https://github.com/DFXswiss/api/commit/a2a8229e8d8e3d8cf26ace24aabefa80395fb3d3#diff-0a3100eb63c4940089757630edf18f46ac140bdb9221e38a28dc593f8deeb4e3
- Guide: https://github.com/DFXswiss/realunit-app/blob/docs/smart-contracts-info/docs/developer-guide.md