Sinon 2.0

Making this a pull request that can be used to track progress and dependencies.

[] Build with browserify
[] Run browserified bundle in browsers
[] Extract fake server
[] Fix deferred tests
[*] Lint
[] Figure out a solution for overridable sinon.log
[] Figure out a solution for overridable sinon.format
[] Figure out an alternative for sinon.config
[] Figure which parts of the public API no longer needs to be public (extend, etc)
[] Figure out which parts to extract as optional add-ons (perhaps assert, test)
[] Use `this` as little as possible
[] Get rid of the sinon.spy() / sinon.spy.create() duality. We don't need sinon.spy.create and the like
[] Do we still want to support mocks? Are people using mocks?
[] Clean up code organization: stream-line file names, flatten directory structure
[] Upgrade Buster to a more recent version