Add rspec-power_assert example

これ標準で入れてよい気がしました

```bash
$ bubsle exec rspec

Sample
pick abd7e28 Add power_assert example
  #say
    Using expect to
      should say 'Hello TDD BootCamp!'
    Using subject it should
      should eq "Yeah! TDD BootCamp!"
    Using power assert
      should
            be asserted by{ sample.say(greeting) == "Wow! TDD BootCamp!" }
                            |      |   |         |
                            |      |   |         true
                            |      |   "Wow!"
                            |      "Wow! TDD BootCamp!"
                            #<Sample:0x007f892bb298a8>

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3 examples, 0 failures
```