Preserve the export statements of specific modules

Need a way to indicate specific modules which will have their export statements preserved through compilation. This will allow the compiler to both process modules and also output modules.

Supports google/closure-compiler-npm#56

I'm envisioning that module rewriting will look something like this:

```js
module$a = function() {};
module$a.foo = 'foo';

// Synthentic extern to be replaced with a module export statement
// before code printing
JSCompiler_preserveExport(module$a);
JSCompiler_preserveExport(JSCompiler_renameProperty('foo', module$a), module$a.foo);
```