Finish functions for an actual blockchain

Need quite a few more functions in order to make this an actual functional blockchain.

- [ ] fn: add a transaction to the current head block
- [ ] fn: mine the current head block and attach a new blank block as head
- [ ] fn: return current head block
- [ ] fn: get current head block hash
- [ ] fn: get entire block chain

Note that I may also want to store the entire blockchain in a JS object keyed by its hash. The block head can be stored separately, since it is not yet complete and needs to be filled with transactions before being mined.