Add a way to estimate gas for the meta transactions and the initial proxy account deploy

[yahgwai]

There's currently no way to get a good estimate of the meta tx gas overhead from the forwarder classes.

We should have:

• some benchmark tests for forward
• a function on the forwarder class - "addOverhead" or something like that, so that users can easily add that
• a function somewhere for calculating the gas required to deploy a new proxy account