Simplify installation instructions

Also, shoulda-matchers needs to be required in rails_helper under RSpec
3.x instead of spec_helper.
Move dev dependencies to Gemfile

Currently some dev dependencies are in the Gemfile, and some are in the
gemspec. The dependencies in the Gemfile are there because they are
using Bundler-specific options, so there is a legitimate reason for them
to be there, however, it's confusing to have gems in two places. So the
common denominator is to have them in the Gemfile.
Extract examples in README to inline documentation
Generate docs using YARD
Publish new docs when releasing a new version