Automatically update manifest plugins permissions on build

The build command will manage permissions on the manifest file, based on what plugins add to config.xml. Example of a permission entry to config.xml:
```xml
    <permission access="readwrite" description="Required for accessing address book." name="contacts" />
```
I'll soon create a PR for cordova-plugins-contact as an usage example.

Some notes on this PR:
- We probably shouldn't be using master branch. I needed the latest build script changes that are on master only.
- defaults.xml is not needed. Cordova will generate a default config.xml for us. Some items were duplicated from what cordova generated and none were actually needed for firefox os.
- Same goes for the manifest.webapp file, we actually generate the file and never use the provided one.
- Manual customizations to the manifest are still possible, only permissions are affected.
- Included xml2js modules as per [the other PR discussion](https://github.com/mozilla-cordova/cordova-firefoxos/pull/1)