added HTML report functionality

HTML report for mutant

This PR has:
- A layout.erb , css and js files to create the html. They are inspired by the simplecov-html files so they should look quite similar.
- A new --reporting option for the command line. If the option is not provided the behavior does not change.
- A reporter/html reporter/html/printer and reporter/html/register classes that are mostly copies from the CLIs, trying to figure out how to get rid of them and if that makes sense. 
- A reporter/html/config file where all the meat is. This file contain a number of helper classes, they can go to different files if you prefer. It gathers the data on that classes and renders the HTML report.

This PR lacks more documentation (please tell me what you want) and specs. 
I'll continue working on it as we discuss the details.