Enable Custom Gas Limit for 'Use Contract'

[jmank88]

When deploying a contract, the user has the option of overriding the default gas limit of 300,000:

When using a contract, the user does not have that option, and the gas limit defaults to 2,000,000:

(example tx)
Users should have the option to override just like when deploying.

An additional 'nice to have' feature would be to use eth_estimateGas either automatically or via a manual 'auto-fill' button to calculate an appropriate gas limit, so they don't have to guess.

[TeamDeedCoin]

It does seem to automatically use 2,000,000 by default for deploying for me. So I had success in launching a contract that uses less than 2mil.

Result:

[treeder]

Must be a big contract! :) fixing now.

[treeder]

PR is in: #43