Add `JSON3.runInContext` and `JSON3.noConflict`
Update Closure Compiler.
Clean up `runInContext`. Simplify `noConflict`.

* `runInContext` now uses `root.Object()` to create a default context,
  avoiding a spec ambiguity in ES 3 that causes some implementations to
  use the native constructor alias to create an object literal. Without
  the workaround, this would fail if `context.Object` were omitted.
* Square bracket notation is used for context imports to ensure that
  Closure Compiler does not munge these property names.
* Fix a typo: `Object` => `objectProto`.
* `noConflict` no longer removes itself after the first invocation.
Add tests for `runInContext` and `noConflict`.
Merging #50.