Update homestead.md

There seems to be a number of 'common' troubleshooting issues with Homestead that people need already.

I've added a troubleshooting section (for future issues) - and included an issue for Windows 7 users that they might need.

See this forum thread for more info: http://laravel.io/forum/05-18-2014-trouble-getting-homestead-to-start-in-windows