Add generators for asset files

Created by: gracewashere

Closes #187 (closed)

Problem:

Many people want to customize the stylesheets and javascripts that get applied to Administrate dashboards.

To do that, they must copy over the default assets from Adminsitrate's source into their own host application, and edit those files to add custom behavior or styles.

This approach is error-prone and not approachable.

Solution:

Add a generator for copying over default styles and javascripts into the user's host application.

Usage:

 # Copy image assets into `app/assets/images/administrate`
rails generate administrate:assets:images

 # Copy javascript assets into `app/assets/javascripts/administrate`
rails generate administrate:assets:javascripts

 # Copy stylesheet assets into `app/assets/stylesheets/administrate`
rails generate administrate:assets:stylesheets

 # Runs all of the above generators
rails generate administrate:assets

ToDo:

• Actually run these generators and edit the files to verify they work.