Allow to use use_tiny_mce from before_filters

Hi!

I've done some modifications to `TinyMCE::Base` so that you can call `use_tiny_mce` from a before filter in addition to the current method. This allows you to do things like that.

```ruby
class PhotosController < ApplicationController
  before_filter :load_resources
  before_filter :set_tiny_mce

  def index
    respond_to do |format|
      format.html
      format.js
    end
  end

  protected

  def load_resources
    @article = Article.find(params[:article_id]
    @photos = @article.photos
  end

  def set_tiny_mce
    use_tiny_mce :only => [:new, :edit], :options => {
      :external_images_list_url => photos_url(@article)
    }
  end
end
```

I hope you find it usefult. Thanks a lot for the plugin!!