[SNAPSHOT] Plugin Beta Changes

[tacticalchihuahua]

This PR is a snapshot of the public PR at https://github.com/MetaMask/metamask-plugin-beta/pull/77 for the purpose of private security review.

Originally from @danfinlay:

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For reviewing relative to the LoginPerSite branch.

Please note many of these changes are totally experimental and intended for removal, such as:

• all default "ambient" APIs (like Fetch and XMLHttpRequest), and so this diff should be compared with an eye of whether the overall approach of isolating plugins and managing the way they communicate with sites and each other is sound.

Included plugin APIs are also each subject to individual consideration before going to production, and are in early stages of implementation. Currently those are:

• The auditAddress API.
• The manageAsset API.
• The registerRpcMethodHandler API.
• Capnode integration via registerApiRequestHandler api.
• (still in progress, probably beyond audit scope) Custom account type API.

You can view known production blockers here.

As always, the wiki is a great place to start to understand the feature and its general API.