Create the API for writing Sidetree transactions

[ebaizel]

https://github.com/decentralized-identity/sidetree-core/blob/master/docs/implementation.md#write-a-sidetree-transaction

Since this may be the first API, this will have a lot of other subtasks including:

• installing KOA (or Express). need to research
• stub out the smart contract integration piece
• choose a test framework (or we can add this as its own issue)