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
Update Appraisals
Remove instances of :nodoc:
Add quick list of matchers to README
Add docs for allow_value
Add docs for allow_mass_assignment_of
Add docs for ensure_inclusion_of
Add docs for ensure_exclusion_of
Add docs for ensure_length_of
Experiment: extract allow_mass_assignment_of docs to own file
Revert "Experiment: extract allow_mass_assignment_of docs to own file"

This reverts commit 9f8e85d8a3500bd641962e51954788f9160d4c98.
Add docs for have_secure_password
Add docs for validate_absence_of
Add docs for validate_acceptance_of
Add docs for validate_confirmation_of
Add docs for validate_numericality_of
Add docs for validate_presence_of
Add docs for validate_uniqueness_of
Add docs for accept_nested_attributes_for