use multiple addon-sdk

This pull request introduce some syntax changes to be able to use multiple addon-sdk in the same Gruntfile.js,
and supports:

- official archives from ftp.mozilla.org
- development version from mozilla/addon-sdk github repo (or github forks)

## Syntax Example

```js
grunt.initConfig({
  "mozilla-addon-sdk": {
    '1_14': {
      options: {
        revision: "1.14"
      }
    },
    'master': {
      options: {
        revision: "master",
        github: true,
        // github_user: "mozilla" // default value
      }
    }
  },
  "mozilla-cfx-xpi": {
    'stable': {
      options: {
        "mozilla-addon-sdk": "1_14",
        extension_dir: "ff_extension",
        dist_dir: "tmp/dist-stable"
      }
    },
    'experimental': {
      options: {
        "mozilla-addon-sdk": "master",
        extension_dir: "ff_extension",
        dist_dir: "tmp/dist-experimental"
      }
    },
  }
});
```