Basic Authority (#991)

* Firt commit.

* First non-functional but correct implementation of BasicAuthority.

Still needs:
- Sealing infrastructure.

* Punch a hole to give miner access to key store.

* Fix test built.

* Basic version of synchronous mining.

This will seal a block whenever a new transaction comes through.
To be made better we need a timer which will wait for one second after the
last block before sealing a new one - better still would be to cooperatively
interleave blocks with other sealing nodes.

* Add tests.

* Fix minor issues from repotting.

* Address grumbles.

ethcore/src/error.rs

@@ -137,6 +137,8 @@ pub enum BlockError {
 	MismatchedH256SealElement(Mismatch<H256>),
 	/// Proof-of-work aspect of seal, which we assume is a 256-bit value, is invalid.
 	InvalidProofOfWork(OutOfBounds<U256>),
+	/// Some low-level aspect of the seal in incorrect.
+	InvalidSeal,
 	/// Gas limit header field is invalid.
 	InvalidGasLimit(OutOfBounds<U256>),
 	/// Receipts trie root header field is invalid.