Move `Contract` class into its own file

[danfinlay]

Issue Type

• [ ] Bug (https://github.com/ethjs/ethjs-contract/blob/master/.github/CONTRIBUTING.md#bug-reports)
• [x] Feature (https://github.com/ethjs/ethjs-contract/blob/master/.github/CONTRIBUTING.md#feature-requests)

Description

In pursuit of #2, I've begun writing a module that imports this one and wraps it with an event-emitter.

Ideally I'd just inherit from the current Contract class, but right now that's not possible, because the Contract class is initialized on each contract creation. This is slightly memory inefficient, but more importantly it prevents me from subclassing the class.

I'm opening this issue to ask if that sounds like an appropriate change to make, before I just open a PR proposing the change.