Fix required keyword behavior when no arguments specified.

Update #respond_to and #construct matchers and specs.

Also create CHANGELOG, update README, and add missing gem versions for
fuubar and pry to gemspec so `gem build` stops whining about it.

Current spec status:
1.9.3: 317 examples, 0 failures in 0.12352 seconds.
2.0.0: 332 examples, 0 failures in 0.14058 seconds.
2.1.0: 355 examples, 0 failures in 0.1177 seconds.
Increment version to 1.0.1 for release.