Introduce brace-expand-join

* I added a test case when the value of [`searchPath` option](https://github.com/jonkemp/gulp-useref#optionssearchpath) is an array.
* Currently, gulp-useref fails to embed assets if `src` property or `href` property starts with parent directory reference, such as `../path/to/script.js`. gulp-useref uses [path.join()](http://nodejs.org/api/path.html#path_path_join_path1_path2) to join glob patterns, but it doesn't work correctly.

  For example, when `searchPath` option is `{foo,foo/bar}` and `script` tag's `src` property is `../modules/script1.js`, We expect `{,foo}/modules/script1.js` as a search path, but the result is `{foo,foo/modules/script1.js`. Actually, [this repo fails the test in such case](https://travis-ci.org/jonkemp/gulp-useref/builds/34882801#L121).

  To fix this problem, I created [brace-expand-join](https://github.com/shinnn/node-brace-expand-join) module. It joins and normalizes glob patterns considreing brace expansion. After adding [brace-expand-join](https://github.com/shinnn/node-brace-expand-join) module, [this repo passes all tests](https://travis-ci.org/jonkemp/gulp-useref/builds/34882904). And brace-expand-join itself is well tested on [*nix](https://travis-ci.org/shinnn/node-brace-expand-join) and [Windows](https://ci.appveyor.com/project/ShinnosukeWatanabe/node-brace-expand-join).