Allow a base url for external links

This commit addressed issue #237 and adds the ability to specify a base url for a given data.json to be
used when constructing urls for external links. For example:

```json
  "external": {
    "data": [
      {
        "base": "http://emberjs.com/api/",
        "json": "http://builds.emberjs.com/tags/v1.5.1/ember-docs.json"
      },
      {
	    "base": "http://emberjs.com/api/",
	    "json": "http://builds.emberjs.com/tags/v1.0.0-beta.6/ember-data-docs.json"
      }
    ]
  }
```
This is not breaking. For example, the following is still valid:
```json
  "external": {
    "data": [
      "http://builds.emberjs.com/tags/v1.5.1/ember-docs.json"
    ]
  }
```