Add support for module references in the serialization

Uses the "%" symbol to reference a module, which is resolved relative
to the module/require of the serialization, and replaced by a module
descriptor as returned by Mr's getModuleDescriptor.

Example:

    // in ui/test.reel/test.html
    "module": {
        value: {"%": "./module"}
    }

    // results in
    {
        id: "ui/test.reel/module",
        require: <require function for package>
        // if the module has been loaded then other properties from
        // https://github.com/montagejs/mr/blob/master/docs/Module-API.md
        // are available
    }
Add ModuleReference module and use in "%" serialization
Add module reference serialization code
Attach a keyComposer to simulated "key" event

I could have sworn the specs were passing when I made the textField
consider the key composer, but they weren't. Specs now pass.
Rename leaveDocument to exitDocument
Fix exitDocument to work bottom-up

When a component leaves the component tree all its children will have their exitDocument called in a bottom-up way.
Handle mouse wheel in flow-translate-composer