Retry report

Admittedly this needs some work - there are no tests and it only works if the documentation formatter is also used.  However, I wanted to send an example of a possible solution for the simplest part of #4 - adding a summary report.

If you run <code>rspec spec/ --format RSpec::Retry::Formatter --format documentation</code> the end of the rspec will be:

<pre>
RSpec Retry Summary:
	should run example until :retry times: 1 out of 3 attempts passed.
	should stop retrying if  example is succeeded: 1 out of 2 attempts passed.
	should clear the let when the test fails so it can be reset: 1 out of 2 attempts passed.
	should not clear the let when the test fails: 1 out of 2 attempts passed.
5 of 5 tests were retried.
0 tests failed all retries.

Finished in 0.0044 seconds
5 examples, 0 failures
</pre>