Understands paths of the other platform when using network feature

When working on Vagrant, the network feature of Guard is quite useful.
However, when the host os is Windows and the guest os is Linux, it doesn't work well because of path handling.
On Linux Ruby environment, Windows paths like "C:/foo/bar" are regarded as relative paths and are expanded like "/home/baz/C:/foo/bar". This causes a trouble because the paths sent from the host machine don't match the converted paths. 
This request fixes the problem and makes Guard work well across platforms.