Move vm.runInThisContext to require.exec

[AMKohn]

In split contexts, like NW.js', vm.runInThisContext evaluates modules in the wrong context. And, due to its location in the code, this behavior is impossible to override.

This commit moves the call to require.exec, where it's overridable, and changes require.makeNodeWrapper to use global.requirejsVars so it doesn't reference a non-existent object when run in NW.js' context.

With this change require can be used in NW.js when exec is overridden like this (although there's a bug with vm in NW.js at the moment so eval has to be used instead):

var vm = require("vm"),
    context = vm.createContext(window);

requirejs.exec = function(text, realPath) {
    text = requirejs.makeNodeWrapper(text);

    return vm.runInContext(text, context, realPath);
};

[digitalkaoz]

Can someone merge this?