fix nocov calculations and issue 146

I found a bunch of methods in the SimpleCov::Result class that seems to be duplication of what's implemented in the FileList class.

* the Result#total_lines method had a bug in it and caused other methods to report inaccurate line numbers and coverage percentages
* the fix: delegate methods to FileList collection to get correct line numbers and percentages
* fixes issue #146
* also, added some documentation about running tests for this project

Let me know if anything looks incorrect. Thanks!