[EXPERIMENTAL] Support test-unit/power_assert instead of RSpec

You can use test-unit/power_assert instead of RSpec with this pull request.

This is a sample test code.

```ruby
require 'serverspec/power_assert'

set :backend, :exec

class ServerspecPowerAssert < Test::Unit::TestCase
  def test_serverspec_power_assert
    assert { file('/tmp').file? }
  end
end
```

The result like this:

![powerassert](https://cloud.githubusercontent.com/assets/3620/4618871/9b84665a-530e-11e4-9002-b0ac598a8d9c.png)

Mine? I don't have anything about that... Anyway, I think it's nice if there is a base class to use instead of extending `Test::Unit::TestCase` directly.