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.
Add YARD as a dev dependency
Remove instances of :nodoc:
Add quick list of matchers to README
Add docs for allow_value
Add docs for allow_mass_assignment_of