Add support for installing assets of tinymce-rails-langs

The problem that assets are deleted by the Capistrano `deploy:assets:clean_expired` task was reported in issue #83 and fixed. However, the same problem also exists in the [tinymce-rails-langs][] gem.

This pull request tries to add support for installing assets of [tinymce-rails-langs][]. It is done by removeing the `ASSETS` constant and save the path to assets as an instance variable.

That is, the path to assets is provided when initialize an new instance of `AssetInstaller`. Therefore, the rake task in [tinymce-rails-langs][] can also use it to install assets.

Please have a look to see if this is an appropriate change.

Thanks! :smile: 

[tinymce-rails-langs]: https://github.com/spohlenz/tinymce-rails-langs