Gas price statistics. (#1291)

* Gas price statistics.

Affects eth_gasPrice.
Added ethcore_gasPriceStatistics.

Closes #1265

* Fix a bug in eth_gasPrice

* Fix tests.

* Revert minor alteration.

* Tests for gas_price_statistics.

- Tests;
- Additional infrastructure for generating test blocks with
transactions.

ethcore/src/verification/verification.rs

@@ -361,7 +361,7 @@ mod tests {
 			nonce: U256::from(2)
 		}.sign(&keypair.secret());
 
-		let good_transactions = [ &tr1, &tr2 ];
+		let good_transactions = [ tr1.clone(), tr2.clone() ];
 
 		let diff_inc = U256::from(0x40);