@JsInterface and magic method injection

Includes experimental support for @JsInterface, demoed at Gwt.create, which replaces overlay types and allows much simpler interaction with javascript apis. (rebased directly on top of https://gwt-review.googlesource.com/#/c/5567/).

This release also includes support for java 8, and magic method injection

These features are all bleeding edge, may have bugs, and may not all make it into gwt master. We will continue to keep a working fork as best we can, but support is a donation of time and not a contractual obligation.