New version of provider-engine includes etherscan-subprovider features required to let Metamask use it.
Hard coded the new version of `web3-provider-engine` even though it is not live on `npm` yet, because it is a dependency of this branch.
I'll deploy to the Chrome store but not merge on Github until that provider-engine is published, because it could break others' dev environments.
Abstract all configuration data into a singleton called `configManager`, who is responsible for reading and writing to the persisted storage (localStorage, in our case).
Uses my new module [pojo-migrator](https://www.npmjs.com/package/pojo-migrator), and wraps it with the `ConfigManager` class, which we can hang any state setting or getting methods we need.
By keeping all the persisted state in one place, we can stabilize its outward-facing API, making the interactions increasingly atomic, which will allow us to add features that require restructuring the persisted data in the long term without having to rewrite UI or even `background.js` code.
All the restructuring and data-type management is kept in one neat little place.
This should make it very easy to add new configuration options like user-configured providers, per-domain vaults, and more!
I know this doesn't seem like a big user-facing feature, but we have a big laundry list of features that I think this will really help streamline.
Also added the hdPath that Christian had told me to our calls to the LightWallet, but this does not seem to have made us generate the same accounts as `testrpc` yet.
Added initial test just to verify we can recover the accounts we generate in this way.
Still need to add compliance test to make sure this interoperates with testrpc's new mnemonic flag.
