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/spec/spec.rs

@@ -244,6 +244,11 @@ impl Spec {
 	pub fn new_test() -> Spec {
 		Spec::load(include_bytes!("../../res/null_morden.json"))
 	}
+
+	/// Create a new Spec which is a NullEngine consensus with a premine of address whose secret is sha3('').
+	pub fn new_null() -> Spec {
+		Spec::load(include_bytes!("../../res/null.json"))
+	}
 }
 
 #[cfg(test)]