Emulation of java.nio package

[zbynek]

This is a placeholder issue for java.nio package emulation.

Since it's not possible to implement it in a way that's both performant and 100% compatible with Java, it is out of scope for GWT.

Users are advised to use a third party library such as https://github.com/Vertispan/gwt-nio or https://github.com/treblereel/gwt-nio. Both are forked from the PlayN project, with slightly different goals - treblereel/gwt-nio has support for more of the java.nio APIs, while Vertispan/gwt-nio has more specific support to what will translate directly to typedarrays that the browser can execute efficiently.

[niloc132]

Edited the description slightly to add a note of https://github.com/treblereel/gwt-nio and a quick summary of their contrasting value.