Create a model for Idea-to-Keyword association

Run generate script for the model IdeaKeyword, add association only to
the Idea model since Keywords hasn't been merged yet.
Add default route

Map it to Idea index page.
Merge branch 'development' of github.com:tktl-app-ideas/appideas into idea_categories

Conflicts:
	config/routes.rb
Implement Idea-to-Keyword relation

Add relation methods to the models.
Add pry-rails gem

Use pry as a default instead of irb.
Merge branch 'idea_categories' into development