It should be possible to run every spec individually

I noticed that some specs can't be run individually, as their dependencies are not correct. This can become rather inconvenient while debugging Administrate. This PR fixes this issue.

A way to check that specs run individually is with the following one-liner:

$ find spec | grep _spec.rb | while read spec; do bundle exec rspec $spec; done

This includes some changes that are not really necessary, but I think make the specs make more sense. I'm referring mainly to things like spec/generators/assets/javascripts_generator_spec.rb, where I make sure that support/generator_spec_helpers is required before the tested code, because I feel that the helpers should be loaded first (like spec_helper or rails_helper) are.