Annotations extensions

Added new extensions to support annotations on classes, functions, and parameters.  Annotations are written in the Java style of <code>@Annotation(param)</code>.  They can appear before a class or function declaration, class elements as well as formal parameters.  The annotations on the class/function declarations and class elements are written out to an <code>annotations</code> property array on the element.  Annotations on formal parameters are written out in a <code>parameters</code> two dimensional array (one array per-parameter) on the parent element.

For example:
```javascript
@Anno
class Test {
  @Anno
  method(@Anno x) {}

  @Anno
  get getter() { return 1; }
}
```

would write the following metadata:
```javascript
Test.annotations = [new Anno];
Test.prototype.method.annotations = [new Anno];
Test.prototype.method.parameters = [[new Anno]];
Object.getOwnPropertyDescriptor(Test.prototype, "getter").get.annotations = [new Anno];
```

Review URL: https://github.com/google/traceur-compiler/pull/660

Closes #660.
Lookup modules in ModuleStore before attempting load.

Define ExportsList as description of exports.
Extend ExportsList to define ModuleDescription and ModuleSymbol.
self-review
Rebase, respond to review and updateSemver