Fix handling of totals in ProgressReporter

Here are my changes per pull request #113 and issue #112.  Instead of repeatedly setting the total every time a test is run, we can set it once when we start the progress bar, presumably because the number of tests should not be changing during a test run.
I originally tried to do both those things.  I agree that it is a bit unnatural this way, but setting total prior to starting actually breaks the output for the empty test class.

![image](https://cloud.githubusercontent.com/assets/1301802/3399267/65644eac-fd3a-11e3-9e0a-af4f895e69a4.png)

Not setting total to nil in the constructor works too, but it defaults the total to 100, which you see briefly prior to starting, and I preferred the current appearance of briefly seeing the uninitialized progress bar instead.