Implement minimal log-in-per-domain on new branch

[danfinlay]

While #714 aspires to a fully shippable version of our per-domain login, we also have a design for a first pass implementation.

This should be implemented on a separate branch, and will undergo user testing before shipping, and we may use this branch to complete #714 rather than shipping in this incomplete form.

This branch should:

• Not inject web3 by default
• Implement a simple postMessage protocol to request the user expose web3 and an account.
• Signing in to a new domain may sign out of all other domains for simplicity (this is the controversial simplification that we want to test before publishing to production).
• The full page UI should no longer reflect the "current selected account" in the MetaMask/logged-in sense, and should allow each tab of itself to freely navigate between accounts, send transactions, etc.

Since this feature is not intended for production yet, styles do not need to be perfect, and what we really want out of this is a usable proof of concept of how the postMessage sign in protocol might work.

There will be open questions in pursuit of this feature. Questions about design should be posted and discussed in #3383, letting this issue focus on implementation.

One open question I just asked there is: When you're signed into site A, then switch to tab at site B, then open MetaMask, what will you see?

Since we don't have a design yet, and I'm a big fan of keeping it simple, and pushing people towards the full-tab interface, I would be happy if the branch implemented something like this:

[danfinlay]

Completed by #4703.