Fixes conversion of paths to web urls for windows

Fixes #3 
Tests failed on windows for me even with this, although it worked for several of my immediate needs yesterday.

I tracked the problem down to grunt.file.isAbsolute path always returning false because the left side comparison contains a drive letter while the right does not. As a result, every path is treated as a relative path. 

I worked around this by stripping the drive letter from the left side with an isAbsolutePath function added to the task and used in place of grunt's. Not ideal, but I needed it for my project. I've forked this code, changed the gruntfile, rewrote tests to reflect my use more closely and tested. My version is passing on Windows 8 and I've been able to build my project with the changes successfully. 

https://github.com/jsimpsonjr00/grunt-filerev-replace/blob/master/tasks/filerev_replace.js#L99