Half implements #175
Things to do:
- Add ability to add tokens to the list.
- Persist the token tab selection (so it is an implicit preference)
- Check what's up with the token-tracker polling, it seems like it is not waiting the interval.
Deriving from the new address image map repository I've added here:
https://github.com/MetaMask/ethereum-contract-icons
With this PR, images for addresses added to that repository will be shown instead of jazzicons in MetaMask.
Fixes#197
Also as a side effect, by creating this `iconFactory.cache` object, we have a convenient place for specifying stock icons for known contracts!
We can just hard-code image addresses in the `ui/lib/icon-factory.js` cache instantiation, and those values will be injected into the identicon image tag `src` attributes.
