Transaction eras (#758)

* Initial groundwork

* A mess.

* Integrate

* Fix tests

* Unit tests

* Tests for unchecked_extrisnic

* fix tab

* Improve binary format.

* fix tests

* Rename extrinsic-pool -> transaction-pool

Closes #770

* Implement unimplemented.

* typo

substrate/srml/staking/src/lib.rs

@@ -52,8 +52,7 @@ use rstd::cmp;
 use runtime_support::{Parameter, StorageValue, StorageMap};
 use runtime_support::dispatch::Result;
 use session::OnSessionChange;
-use primitives::traits::{Zero, One, Bounded, OnFinalise,
-	As, Lookup};
+use primitives::traits::{Zero, One, Bounded, OnFinalise, As};
 use balances::{address::Address, OnDilution};
 use system::ensure_signed;