Add ability to invoke as a read-only simulate transaction only

[leighmcculloch]

trafficstars

We need to be able to invoke contracts in readonly mode, where we expect no affect to take place.

We should add an option to the invoke command to allow a user to choose to be doing this.

In that mode there shouldn't be a need for a secret key or a network passphrase, and the invocation should only run simulation, it shouldn't submit the actual invocation to the network.

cc @tyvdh @2opremio