Change #execute! to #call so that pay_dirt service

  - objects can stand in for procs or vice versa
  - update README with some info explaining pay_dirt's name
Don't default to minitest

  - ... thereby not appending to minitest_helper.rb by default
Bump version
Merge branch 'master' of github.com:rthbound/pay_dirt

* 'master' of github.com:rthbound/pay_dirt:
  Update README.md

Conflicts:
	README.md