v2.0 - One Happy Family

This is a big one! These are the most significant changes:

• Combined the AltspaceSDK and aframe-altspace-component repositories into one.
• Ported the entire SDK codebase to the new ECMAScript 2015 syntax.
• New package build system based on ES2015 modules.
• Reformatted, cleaned, and improved documentation for both APIs.
• All JS behaviors are now classes, so the new keyword is required when instantiating them. This is reflected in the examples.
• All A-Frame components are classes in addition to registered components. They are accessible through the altspace.components namespace. They are not usable in JS as-is, but might be useful for third parties to replicate the functionality.
• Updated the official Three.js version recommendation to r84.