Support @throws tag

It works similar with the `@return` tag. This means that a type is optional. An actual example is like the following. And, this resolves #135 finally.

```js
/**
 * @method getName
 * @throws {Error} Throws an exception error.
 */
function getName() {
    // do something
}
```

This is a new feature, so I wish I could get any reviewers.

/cc @juandopazo @caridy 
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