Fixed watchdir option

I think current watchdir option doesn't work as it should (please correct me if I'm missing something).

My recent project was consisted of many different modules each of them having it's own tests suite.
I've tried to use watchdir option but it didn't work as I expected.

I've created sample project and described steps to reproduce the issue and fix for the issue with my forks (https://github.com/lesniakania/guard and https://github.com/lesniakania/guard-rspec):

https://github.com/lesniakania/multiple-modules-with-guard
relative_path_from can fail with ArgumentError on Windows (http://stackoverflow.com/questions/669417/ruby-relative-path-from-call-on-windows) - in that case, full path should be used (e.g. with someone watches d:\data and pwd is c:\project)