Implement GWT.getModuleBaseURL

[niloc132]

Main issues with this are how to handle being invoked in a non-browser situation, and how to be flexible when loaded from an iframe or the like.

Easiest way to do this is to probably to run some .native.js on startup to find the <script> tag currently being invoked and read the path string off of it, but allow some other string property to override this behavior to support the above use cases.

GWT2 compat probably comes from reading out of $moduleBase.

[niloc132]

Be sure to see https://github.com/Vertispan/gwt-core/issues/2#issuecomment-424769183 - this task also matters for former public resources, for static files to use in testing.