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: Added GasPriceChange event
* eth: When one of the DB flush methods error a fatal error log message
is given. Hopefully this will prevent corrupted databases from
occuring.
* miner: remove transactions with low gas price. Closes#906, #903
