Transform absolute file paths to file URI scheme.

On Windows, phantomjs appears to have issues processing relative paths for
referenced files (e.g., script/style refs) when its webpage is created via
an absolute path. I was unable to find a relevant issue on GitHub.

`gulp.src` produces absolute paths in some cases/environments, which will
trigger the above phantomjs issue when using this plugin.

This fix will determine if a file path is absolute and, if so, will
transform it to the file URI scheme (e.g., 'file:///c:/path/to/test.html')
which phantomjs *can* handle. Relative file paths remain untouched.

A test case demonstrating the behaviour was added.
Merge pull request #8 from jgoz/relative-path-win-issue

Transform absolute file paths to file URI scheme