core, core/types: readd transactions after chain re-org

Added a `Difference` method to `types.Transactions` which sets the
receiver to the difference of a to b (NOTE: not a **and** b).

Transaction pool subscribes to RemovedTransactionEvent adding back to
those potential missing from the chain.

When a chain re-org occurs remove any transactions that were removed
from the canonical chain during the re-org as well as the receipts that
were generated in the process.

Closes #1746

core/chain_makers_test.go

@@ -42,7 +42,7 @@ func ExampleGenerateChain() {
 	)
 
 	// Ensure that key1 has some funds in the genesis block.
-	genesis := WriteGenesisBlockForTesting(db, addr1, big.NewInt(1000000))
+	genesis := WriteGenesisBlockForTesting(db, GenesisAccount{addr1, big.NewInt(1000000)})
 
 	// This call generates a chain of 5 blocks. The function runs for
 	// each block and adds different features to gen based on the