Don't generate dashboards for non-constant models

Created by: gracewashere

Fixes #145 (closed) and #219 (closed)

Problem:

When the same has_and_belongs_to_many relationship is defined twice, Rails generates an unnamed subclass of ActiveRecord::Base. This is probably a bug in Rails.

The unnamed subclass shows up like this:

> ActiveRecord::Base.descendants.last
 #<Class:0x007fddea1bdf08> (call '#<Class:0x007fddea1bdf08>.connection' to establish a connection)

When the install generator tries to populate the DashboardManifest with all of the ActiveRecord models, it trips up on unnamed models, and generates a manifest with invalid syntax.

This results in a NameError for the user when they run the administrate:install generator:

/gems/administrate-0.1.0/lib/generators/administrate/dashboard/dashboard_generator.rb:87:
in `const_get': wrong constant name #<class:0x007fcf52bfe248> (NameError)

This error most often pops up in applications that are using the spring gem.

Solution:

• Add a test case that reproduces the error by defining duplicate has_many associations.
• In the install generator, ignore any models that are not constants. We determine constants by checking that the model's #name matches its #to_s.
• Print a warning for the ignored models.