Adding links to GitHub repo to Makefile.PL

This addition will add link 'Clone repository' on metacpan page.

Here is an example how it will look like:

![screen shot 2014-04-29 at 11 22 34](https://cloud.githubusercontent.com/assets/47263/2826412/27de51fc-cf6f-11e3-9342-bdabf51e05cb.png)