Now any strategy for importing a private key that can be described as a pure function can be very easily turned into a MetaMask import strategy.
I've created a generic and reusable UI action called `importNewAccount(strategy, args)`.
The `strategy` is a unique identifier defined in `app/scripts/account-import-strategies`, and the `args` will be passed to the member of the `strategies` array whose key matches the strategy string.
Strategies return private key hex strings, and are used by the metamask-controller to create a new keyring, and select that new account, before calling back.
This also implements @frankiebee's idea of showing the imported account when it's been imported (my oversight!).
This commit only moves us to this architecture, keeping feature parity for private key import, but has some untested code for importing geth-style JSON files as well!
Fixes#1021
A top-right menu item now allows `Account Import`. It has a menu (with one item for now) that allows importing a private key string.
Errors are displayed, and a success navigates the user to their account list, where the imported account is labeled `LOOSE`.
