[frontport] CLI to specify queue ordering strategy (#2494) (#2623)

* CLI to specify queue ordering strategy (#2494)

* Alter gas priorities to include gas_price also

* CLI options and tests

* Adding ordering by gas

* whitespace

Conflicts:
	ethcore/src/miner/miner.rs
	ethcore/src/miner/mod.rs
	ethcore/src/miner/transaction_queue.rs
	parity/cli/usage.txt
	parity/configuration.rs

* fix build