Solve issue with classes of path attributes in Dashing config.

Hi, all. 

I had an issue while running `rails g dashing:widget simple_widget`. Here it is: `dashing-rails-2.3.2/lib/generators/dashing/widget_generator.rb:14:in `widget': undefined method `join' for "app/assets/stylesheets/dashing":String (NoMethodError)`. The problem was that `widgets_js_path`and `widgets_css_path` was expected to be `Pathname` classes. This issue is related to #43. 

I decided to solve this issue the way I did in this PR. This allows to accept attributes with Proc, String and Pathname classes, but always return Pathname class. 

If you accept this solution, I will add tests and a little bit changes.
this one should be changed too