2.0.0-beta.8 upgrade PR

[guybedford]

This is a work-in-progress PR for the 2.0.0-beta.8.

Features:

• Updates to the latest resolver (Support exports main and dot main jspm-resolve#15), which includes support for package.json "exports" and full compatibility with the latest Node.js --experimental-modules resolver. This means we guarantee a package resolution that works in Node.js works in jspm.
• Updates the linking and checkout workflows. jspm checkout pkg now automatically clones packages into jspm_packages, and maintains the clone through subsequent installs. If the package is altered such that the package.json version changes, subsequent jspm update calls will result in the jspm_packages version being updated to that. This provides a comprehensive submodules-style development workflow.
• ES Module conversion is no longer applied to packages running in Node.js. This way full npm compatibility is provided.
• jspm install jspm --dev can now be used to self-host a local jspm version which will be used to operate on the package itself. This replaces the previous npm install jspm --save-dev version locking jspm used previously.
• Various Windows binary fixes - eg jspm install mocha && jspm bin mocha now works fine in Windows.
• Adds support for --force to installs to do full validation and clearing of jspm_packages
• All GitHub installs are now plain git installs aliased into npm packages. Thus only one registry is supported again.
• global bin installs now by default check for a local jspm
• local bin installs now by default use the local resolver
• jspm loads its own dependencies using a submodules-style workflow, making it full self-hosted
• zero-arguments jspm build as a full package optimization

In progress:

• jspm build to handle "exports"
• Optimize condition handling in builds, including output "exports".
• Update jspm map to handle "exports"