EIP-1102 Dapp interaction

[germartinez]

Additions:

• ethereum.enable always returns the current account on the browser extension. There is no need to return an empty provider and wait for the user to approve the current Dapp as we already have implemented the whitelisting feature.
• Refactor background script in different controllers and components inside the pop-up window
• Temporary support for syncronous eth_accounts and eth_coinbase calls (compatibility issues with Dapps)
• Temporarily added the functions isApproved and isUnlocked that Metamask uses (compatibility issues with Dapps)

[tschubotz]

Just to clarify:

ethereum.enable always returns the current account on the browser extension.

We only do that when the site is whitelisted, right? Same for the other functions, like isApproved, right?

Besides that: How do you want me to review this PR? I'm not sure if I'm the right person to review the code. ;)